Alignment systems and methods for lithographic systems

ABSTRACT

An alignment system for a lithographic apparatus has a source of alignment radiation; a detection system that has a first detector channel and a second detector channel; and a position determining unit in communication with the detection system. The position determining unit is constructed to process information from said first and second detector channels in a combination to determine a position of an alignment mark on a work piece, the combination taking into account a manufacturing process of the work piece. A lithographic apparatus has the above mentioned alignment system. Methods of alignment and manufacturing devices with a lithographic apparatus use the above alignment system and lithographic apparatus, respectively.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. application Ser. No.11/294,475, filed Dec. 6, 2005, now abandoned which is a divisional ofU.S. application Ser. No. 10/665,404, filed Sep. 22, 2003, now U.S. Pat.No. 7,332,732 which claims priority to U.S. Provisional Application No.60/411,861, filed Sep. 20, 2002, U.S. Provisional Application No.60/413,601, filed Sep. 26, 2002, European Application No. 03075954.2,filed Apr. 1, 2003 and European Application No. 03076422.9, filed May12, 2003. The entire contents of these applications are incorporatedherein by reference.

BACKGROUND OF THE INVENTION

1. Field of Invention

This invention relates to an alignment system for a lithographicprojection apparatus, and a lithographic projection apparatus havingsuch an alignment system, and more particularly to an alignment systemthat can detect the position of an alignment mark using at least twoseparate signals detected substantially in parallel and/or detect theposition of a multi-target mark.

2. Discussion of Related Art

Lithographic projection apparatuses are essential components for themanufacture of integrated circuits and/or other microdevices. With theaid of such an apparatus, a number of masks having different maskpatterns are successively imaged at a precisely aligned position onto asubstrate such as a semiconductor wafer or an LCD panel. The substratemust undergo the desired physical and chemical changes between thesuccessive images that have been aligned with each other. The substrateis removed from the apparatus after it has been exposed with a maskpattern, and, after it has undergone the desired process steps, thesubstrate is replaced in order to expose it with an image of a secondmask pattern, and so forth, while it must be ensured that the images ofthe second mask pattern and the subsequent mask patterns are positionedaccurately with respect to the substrate. To this end, the lithographicprojection apparatus is provided with an optical alignment system withwhich alignment marks on the substrate are aligned with respect toalignment marks on the mask.

A lithographic apparatus may not only be used for the manufacture of ICsbut also for the manufacture of other structures having detaileddimensions of the order of 1 micrometer, or smaller. Examples arestructures of integrated, or plenary, optical systems or guiding anddetection patterns of magnetic domain memories, micro-electromechanicalsystems (MEMS), and structures of liquid crystal display panels. Also inthe manufacture of these structures, images of mask patterns must bealigned very accurately with respect to a substrate.

The lithographic projection apparatus may be a stepping apparatus or astep-and-scan apparatus. In a stepping apparatus, the mask pattern isimaged in one shot on an IC area of the substrate. Subsequently, thesubstrate is moved with respect to the mask in such a way that asubsequent IC area will be situated under the mask pattern and theprojection lens system and the mask pattern is imaged on the subsequentIC area. This process is repeated until all IC areas of the substrateare provided with a mask pattern image. In a step-and-scan apparatus,the above-mentioned stepping procedure is also followed, but the maskpattern is not imaged in one shot, but via scanning movement. Duringimaging of the mask pattern, the substrate is moved synchronously withthe mask with respect to the projection system and the projection beam,taking the magnification of the projection system into account. A seriesof juxtaposed partial images of consecutively exposed parts of the maskpattern is imaged in an IC area. After the mask pattern has beencompletely imaged in an IC area, a step is made to a subsequent IC area.A possible scanning procedure is described in the article: “Sub-micron1:1 Optical Lithography” by D. A. Markle in the magazine “SemiconductorsInternational” of May 1986, pp. 137-142.

U.S. Pat. No. 5,243,195 discloses an optical lithographic projectionapparatus provided with an alignment system and intended for themanufacture of ICs. This alignment system comprises an off-axisalignment unit for aligning a substrate alignment mark with respect tothis alignment unit. In addition, this alignment system comprises asecond alignment unit for aligning a substrate mark with respect to amask mark via the projection lens (TTL). Alignment via the projectionlens (on-axis alignment) is frequently used in many current generationof optical lithographic projection apparatuses and provides theadvantage that the substrate and the mask can be aligned directly withrespect to each other. When the off-axis alignment method is used, thebaseline offset as described in U.S. Pat. No. 5,243,195 must be takeninto account. However, with the continued decrease in the size ofcomponents on ICs and the increase in complexity, on-axis alignmentsystems have proven to be difficult to improve sufficiently to achievethe require precision and accuracy.

In connection with the increasing number of electronic components perunit of surface area of the substrate and the resultant smallerdimensions of these components, increasingly stricter requirements areimposed on the accuracy with which integrated circuits are made. Thepositions where the successive masks are imaged on the substrate musttherefore be fixed more and more accurately. In the manufacture ofnew-generation ICs with smaller line widths, the alignment accuracy willhave to be improved or, in other words, it must be possible to detectsmaller deviations so that the resolving power of the alignment systemmust be increased. On the other hand, stricter requirements must also beimposed on the flatness of the substrate due to the required highernumerical aperture (NA) of the projection lens system in the case ofdecreasing line widths. The depth of focus of this system decreases asthe NA increases. Since some image field curvature occurs at the desiredrelatively large image field of the projection lens system, there ishardly any room left for unevenness of the substrate. To obtain thedesired flatness of the substrate, it has been proposed to polish thesubstrate by the chemical mechanical polishing (CMP) process between twoconsecutive exposures with different mask patterns in the projectionapparatus. However, this polishing process affects the accuracy of theon-axis alignment method. In this method, a grating is used as asubstrate alignment mark and the sub-beams diffracted in the first orderby this grating are used for imaging the substrate mark on the reticlemark. In this process, it is assumed that the substrate is alignedcorrectly with respect to the reticle when the point of gravity of thesubstrate grating mark is aligned with respect to the point of gravityof the reticle alignment mark. In that case it has been assumed that thepoint of gravity for each grating mark coincides with the geometricalcenter of the grating. However, the CMP process renders the substrategrating mark asymmetrical so that this alignment method is no longerreliable. In addition, the various processing steps contribute tochanges in the alignment marks including introducing asymmetries andchanges in the effective depth of the grooves of the substrate gratingmarks. Other processing steps and/or methods often introduce differenttypes of errors. For example, the Cu-damascene process tends tointroduce alignment errors in a random distribution across an ICsurface. With the decrease in size and increase in complexity ofstructures built by lithographic techniques there is a continued demandto improve alignment accuracy. Without the improvement of alignmentaccuracy, improvements in resolution cannot be utilized. In addition,the increased complexity of the micro- devices places greater demand fortechniques to control and minimize the number of substrates that have tobe discarded during the manufacturing process due to alignment errors.

SUMMARY

It is thus an object of the present invention to provide an alignmentsystem for a lithographic projection apparatus that has improvedalignment accuracy and/or robustness.

In order to achieve these and other objectives of this invention analignment system for a lithographic apparatus is provided with a sourceof alignment radiation; a detection system comprising a first detectorchannel and a second detector channel; and a position determining unitin communication with said detection system. The position determiningunit processes information from the first and second detector channelsin combination to determine a position of an alignment mark on a firstobject relative to a reference position on a second object based on thecombined information.

According to another embodiment of this invention, a lithographicprojection apparatus has a source of illumination radiation; a substratestage assembly arranged in a radiation path of the source ofillumination radiation; a reticle stage assembly arranged in theradiation path of the source of illumination radiation between thesource and the substrate stage assembly; a projection system arrangedbetween the reticle stage assembly and the substrate stage assembly; andan off-axial alignment system arranged adjacent to the projection systemand proximate the substrate stage assembly. The off-axial alignmentsystem has a source of alignment radiation; a detection systemcomprising a first detector channel and a second detector channel; and aposition determining unit in communication with the detection system.The position determining unit processes information from the first andsecond detector channels in combination to determine a position of analignment mark on a first object relative to a reference position on asecond object based on the combined information. The alignment systemmay be located away from said radiation path of illumination radiation.All that is required is that alignment radiation from the alignmentsystem is able to reach the substrate stage assembly.

According to another embodiment of this invention, a method of aligninga workpiece for the manufacture of a microdevice includes forming amulti-target alignment mark on the workpiece; scanning the multi-targetalignment mark with an alignment system having a plurality of detectors,wherein a first target of the multi-target alignment mark is adapted tobe detected by a first detector of the plurality of detectors of thealignment system, and a second target of the multi-target alignment markis adapted to be detected from a second detector of the plurality ofdetectors of the alignment system; receiving a first signal from thefirst detector of the alignment system in response to the first targetof the multi-target alignment mark; receiving a second signal from thesecond detector of the alignment system in response to the second targetof the multi-target alignment mark; and determining a position of thealignment mark on the workpiece relative to a predetermined referenceposition based on information from the first and second signals.

According to another embodiment of this invention a method of capturingan alignment mark on a workpiece for the manufacture of a microdevicewithin a measurement region includes forming a multi-grating alignmentmark on the workpiece; scanning the multi-grating alignment mark with analignment system having a plurality of detectors; selecting first andsecond gratings from said multi-grating alignment mark; comparing afirst substantially periodic signal from the first grating of themulti-grating alignment mark from a first one of the plurality ofdetectors to a second substantially periodic signal from the secondgrating of the multi-grating alignment mark from a second one of theplurality of detectors; and determining a capture range based on thecomparing. The first grating can be a diffraction-order-enhancinggrating that enhances an order greater than first order and the secondgrating can be a diffraction-order-enhancing grating that enhances anorder greater than first order.

According to another embodiment of this invention, an alignment mark foruse in the manufacture of a micro-device has a first target that has afirst detection pattern; and a second target that has a second detectionpattern. The first target is adapted to be detected by a first detectorand the second target is adapted to be detected by a second detector.

According to another embodiment of this invention, adiffraction-order-enhancing alignment mark for use in the manufacture ofa micro-device has a diffraction grating pattern that has a periodicpattern and a sub-pattern. The diffraction-order-enhancing gratingenhances a strength of a diffracted beam of non-zero order.

According to another embodiment of this invention, an alignment mark foruse in the manufacture of a microdevice has a target that has adetection pattern and a processing pattern. The processing pattern has astructure that changes under micro-device processing in correspondencewith changes to said micro-device during manufacture.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an embodiment of a step-and-scan projection apparatus withthe various measuring systems;

FIG. 2 shows an embodiment of a substrate alignment mark;

FIG. 3 shows an embodiment of a double alignment unit for aligning amask mark and a substrate mark with respect to each other;

FIG. 4 is a schematic illustration of a lithographic apparatus that hasan off-axial alignment system according to an embodiment of thisinvention;

FIG. 5 shows an embodiment of an off-axis alignment unit according tothe invention;

FIG. 6 shows a plate with reference gratings used in an embodiment ofthe invention;

FIG. 7 shows the deflection by the wedge elements in an embodiment ofthe invention;

FIG. 8 shows a preferred arrangement of the first and second lenssystems in the alignment unit according to an embodiment of theinvention;

FIG. 9 shows a series of wedge-shaped plates to be used as the structureof deflection elements in a second embodiment of the alignment unit;

FIG. 10 illustrates how this series deflects a sub-beam;

FIG. 11 shows the positions of the sub-beams in the plane of this platein an embodiment of the alignment unit in which alignment radiation withtwo wavelengths is used;

FIG. 12 shows an embodiment of the alignment unit in which twowavelengths are used;

FIG. 13 shows a preferred beam splitter for use in this embodiment;

FIG. 14 shows the position of the alignment unit with respect to theprojection lens and the substrate;

FIGS. 15A-15D illustrate asymmetric damage to alignment marks due to thetungsten chemical mechanical polishing (W-CMP) process;

FIG. 16 shows alignment errors due to the W-CMP process as a function ofdiffraction order of the detection channel;

FIG. 17 shows the alignment errors of FIG. 16, plotted a different way;

FIG. 18 shows false expansion errors due to the W-CMP and aluminumphysical vapor deposition (Al-PVD) process as a function of periodicityfor four semiconductor wafers;

FIG. 19 shows false rotation errors due to the W-CMP and AL-PVD processas a function of periodicity for four semiconductor wafers;

FIG. 20 is a schematic illustration of X and Y two-target alignmentmarks written in scribe lines of a semiconductor wafer;

FIG. 21 illustrates output signals from seven diffractive order channelsfor one of two wavelengths in an off-axial alignment system according toan embodiment of this invention;

FIG. 22 is a schematic illustration of a four-target alignment mark,suitable for forming in scribe lines of semiconductor wafers, accordingto an embodiment of this invention;

FIGS. 23A-23D are schematic, cross sectional views of portions ofdiffraction-order enhancing gratings suitable for targets in alignmentmarks according to embodiments of this invention;

FIG. 24 is a perspective view illustrating a substructure of analignment grating that has a processing structure;

FIG. 25 is a schematic illustration for explaining the concept of analignment capture system and method according to an embodiment of theinvention;

FIGS. 26A, 26B and 26C are schematic illustrations of multi-targetalignment marks that have process specific targets according to anembodiment of this invention;

FIG. 27 is a schematic illustration of a multi-target alignment markaccording to another embodiment of this invention;

FIG. 28 illustrates a semiconductor processing system according to anembodiment of this invention; and

FIG. 29 illustrates processing quality control according to anembodiment of this invention.

DETAILED DESCRIPTION

Methods and devices according to this invention will now be describedwith reference to particular embodiments by way of example. The broadconcepts of this invention are not limited to only these specificallydescribed embodiments. The invention will be described with reference toan alignment system for a photolithography system that includes both anon-axis (also referred to as “axial”) and an off-axis (“off-axial”)alignment system that can be used in combination to obtain the eventualalignment of a mask with respect to a substrate (“workpiece”). The axialalignment system may have a separate source of radiation to illuminatealignment marks, such as in through-the-lens (TTL) orthrough-the-reticle (TTR) systems, or it may employ the same radiationas the exposure radiation. The following example will describe a TTLsystem in combination with an off-axial system (OAS) as an embodiment ofthis invention. Furthermore, the invention envisions application tophotolithography systems that have refraction projection systems as wellas to other types of lithography systems that use shorter wavelengths ofelectromagnetic radiation than currently employed, systems which usereflective and/or diffraction imaging optics, and/or systems which useother types of radiation such as charged-particle beams, e.g., electronbeams that are imaged with magnetic, electromagnetic, and/orelectrostatic imaging optics. Embodiments of this invention alsoenvision integrating the alignment process of the lithography systemswith other components of an Automated Process Control (APC) system suchas a metrology tool that is used to measure the accuracy of an exposureof a photo-resist prior to further processing.

We now describe an optical lithographic projection apparatus that has anon-axis alignment unit and other measuring systems as an example of asystem that may incorporate an embodiment of the instant invention.

FIG. 1 shows diagrammatically the optical elements of an embodiment ofsuch an apparatus for step-and-scan imaging a mask pattern on asubstrate. This apparatus comprises, as its main component, a projectioncolumn incorporating a projection system PL. Situated above this systemis a mask holder MH for a mask MA in which the mask pattern C to beimaged is provided. The mask holder is part of a mask stage MT. Asubstrate stage WT is arranged below the projection lens system PL. Thistable comprises a substrate holder WH for a substrate W provided with aphotosensitive layer. The mask pattern C must be imaged a number oftimes in the photosensitive layer, every time in a different area, an ICarea W_(d). The substrate table is movable in the X and Y directions sothat, after imaging the mask pattern in a first IC area, a subsequent ICarea can be positioned under the mask pattern.

The apparatus further comprises an illumination system which is providedwith a radiation source LA, for example a Krypton-Fluoride Excimer laseror a mercury lamp, a lens system LS, a mirror RE and a condenser lensCO. The projection beam PB supplied by the illumination systemilluminates the mask pattern C. This pattern is imaged by the projectionlens system PL on an IC area of the substrate W. The projection lenssystem has, for example, a magnification M=¼, a numerical apertureNA=0.6 and a diffraction-limited image field with a diameter of 22 mm.

The apparatus is further provided with a number of measuring systems,namely a system for aligning the mask MA and the substrate W withrespect to each other in the XY plane, an interferometer system fordetermining the position and orientation of the substrate holder andhence of the substrate, and a focus error detection system fordetermining a difference between the focal or image plane of theprojection lens system PL and the surface of the substrate W. Thesemeasuring systems are parts of servo systems which comprise electronicsignal-processing and control circuits and drivers, or actuators, withwhich the position and orientation of the substrate and the focusing canbe corrected with reference to the signals supplied by the measuringsystems.

The alignment system makes use of two alignment marks M₁ and M₂ in themask MA shown in the top right part of FIG. 1. As is shown in FIG. 2,these marks may be diffraction gratings, but may alternatively be othermarks such as squares or strips which are optically distinguished fromtheir surroundings. The alignment marks may be two-dimensional, i.e.they extend in two mutually perpendicular directions, the X and Ydirections in FIG. 1, or may be used in conjunction with other marks toextend in both X and Y directions. The substrate W, for example asemiconductor substrate, has at least two alignment marks, which may betwo-dimensional diffraction gratings, two of which, P₁ and P₂, are shownin FIG. 1. The marks P₁ and P₂ are situated outside the IC areas on thesubstrate W. The grating marks P₁ and P₂ are preferably implemented asphase gratings and the grating marks M₁ and M₂ are preferablyimplemented as amplitude gratings. Other types of alignment marks may beprovided along scribe lines between adjacent circuits.

FIG. 1 shows a special embodiment of an on-axis alignment unit, namely adouble alignment unit in which two alignment beams b and b′ are used foraligning the substrate alignment mark P₂ on the mask alignment mark M₂,and for aligning the substrate alignment mark P₁ on the mask alignmentmark M₁, respectively. The beam b is reflected by a reflecting element30, for example a mirror, towards the reflecting surface 27 of a prism26. The surface 27 reflects the beam b towards the substrate alignmentmark P₂ which sends a part of the radiation as beam b₁ to the associatedmask mark M where an image of the mark P₂ is formed. Situated above themark M₂ is a reflecting element 11, for example a prism, which directsthe radiation that passes by the mark M₂ towards a radiation-sensitivedetector 13.

The second alignment beam b′ is reflected by a mirror 31 towards areflector 29 in the projection lens system PL. This reflector sends thebeam b′ to a second reflecting surface 28 of the prism 26, which surfacedirects the beam b′ onto the substrate alignment mark P₁. This markreflects a part of the radiation of the beams b′ as beam b′₁ to the maskalignment mark M₁ where an image of the mark P₁ is formed. The radiationof the beam b′₁ passing through the mark M₁ is directed by a reflector11′ towards a radiation sensitive detector 13′.

FIG. 2 shows an embodiment of one of the two identical substrate marks,in the form of phase gratings, on a larger scale. Such a grating mayconsist of four subgratings P_(1,a), P_(1,b), P_(1,c) and P_(1,d), twoof which, P_(1,b) and P_(1,d), serve for alignment in the X directionand the two other ones, P_(1,a) and P_(1,c) serve for alignment in the Ydirection. The two sub-gratings P_(1,b) and P_(1,c) have a gratingperiod of, for example 16 μm and the sub-gratings P_(1,a) and P_(1,d)have a grating period of, for example 17.6 μm. Each sub-grating may havea dimension of, for example 200×200 μm. An alignment accuracy which, inprinciple, is smaller than 0.1 μm can be achieved with this grating markand a suitable optical system. By choosing different grating periods,the capture range of the alignment unit can be enlarged. This range is,for example 40 μm.

FIG. 3 shows the optical elements of a slightly modified alignment unitin greater detail. The double alignment unit comprises two separate andidentical alignment systems AS₁ and AS₂ which are positionedsymmetrically with respect to the optical axis AA′ of the projectionlens system PL. The alignment system AS₁ is associated with the maskalignment mark M₂ and the alignment system AS₂ is associated with themask alignment mark M₁. The corresponding elements of the two alignmentsystems are denoted by the same reference numerals, those of the systemAS₂ being primed so as to distinguish them from those of the system AS₁.

The structure of the system AS₁ will now be described, as well as theway in which the mutual position of the mask mark M₂ and, for example,the substrate mark P₂ is determined with this system.

The alignment system AS₁ comprises a radiation source 1, for example aHelium-Neon laser emitting an alignment beam b. This beam is reflectedto the substrate W by a beam splitter 2. The beam splitter may consistof a semi-transparent mirror or a semitransparent prism but ispreferably constituted by a polarization-sensitive splitting prism 2preceding a λ/4 plate 3, in which λ is the wavelength of the beam b. Theprojection lens system PL focuses the beam b in a small radiation spotV, having a diameter of the order of 1 mm, on the substrate W. Thissubstrate reflects a part of the beam as beam b₁ towards the mask MA.The beam b₁ traverses the projection lens system PL, which system imagesthe radiation spot V on the mask. Before the substrate is arranged inthe projection apparatus, it has been pre-aligned in a pre-alignmentstation coupled to the apparatus, for example the station described inEP Patent Application 0 164 165, such that the radiation spot V islocated on the substrate mark P₂. This mark is then imaged on the maskmark M₂ by the beam b₁. With the magnification M of the projection lenssystem being taken into account, the dimension of the mask mark M₂ isadapted to that of the substrate mark P₂ so that the image of the markP₂ coincides accurately with the mark M₂ when the two marks are mutuallypositioned correctly.

On its path to and from the substrate W, the beams b and b₁ havetraversed the λ/4 plate 4 twice, whose optical axis is at an angle of45° to the direction of polarization of the linearly polarized beam bcoming from the source 1. The beam b₁ passing through the λ/4 plate thenhas a direction of polarization which is rotated 90° with respect to thebeam b so that the beam b₁ is passed by the polarization splittingprism. The use of the polarization splitting prism in combination withthe λ/4 plate provides the advantage of a minimal radiation loss whencoupling the alignment beam into the radiation path of the alignmentsystem.

The beam b₁ passed by the alignment mark M₂ is reflected by a prism 11and directed, for example by a further reflecting prism 12 towards aradiation-sensitive detector 13. This detector is, for example acomposite photodiode having, for example four separateradiation-sensitive areas in conformity with the number of sub-gratingsshown in FIG. 2. The output signals of the detector areas are a measureof coincidence of the mark M₂ with the image of the substrate mark P₂.These signals can be processed electronically and used for moving themask and the substrate with respect to each other by means of drivingsystems (not shown), such that the image of the mark P coincides withthe mark M. An automatic alignment system is thus obtained.

A beam splitter 14 in the form of, for example a partially transparentprism splitting a portion of the beam b₁ as beam b₂ may be arrangedbetween the prism 11 and the detector 13. The split beam is incidentvia, for example two lenses 15 and 16 on a television camera 17 which iscoupled to a monitor (not shown) on which the alignment marks P₂ and M₂are visible to an operator of the projection apparatus. This operatorcan then ascertain whether the two marks coincide and move the substrateW by means of manipulators so as to cause the marks to coincide.

Analogously as described hereinbefore for the marks M₂ and P₂, the marksM₁ and P₂ and the marks M₁ and P₁ can be aligned with respect to eachother. The alignment system AS₂ is used for the last-mentioned twoalignments.

For further particulars about the construction and the alignmentprocedure of the double alignment unit, reference is made to U.S. Pat.No. 4,778,275, which is incorporated herein by reference.

The embodiment of the on-axis alignment unit shown in FIG. 1 isparticularly suitable for an apparatus in which a projection beam PBhaving a short wavelength, for example 248 nm, and an alignment beamhaving a considerably larger wavelength, for example 633 nm, are used.

Since the projection lens system is designed for the wavelength of theprojection beam PB, differences occur when this system PL is used forimaging the alignment marks P₁, P₂ and M₁ and M₂ on each other by meansof the alignment beam. For example, the substrate alignment marks willnot be situated in the plane of the mask pattern in which the maskalignment marks are situated, but will be imaged at a given distancetherefrom, which distance depends on the difference between thewavelength of the projection beam and the alignment beam and thedifference between the refractive indices of the material of theprojection lens elements for the two wavelengths. If the projection beamhas a wavelength of, for example 248 nm and the alignment beam has awavelength of 633 nm, this distance may be 2 mm. Moreover, due to saidwavelength difference, a substrate alignment mark is imaged on a maskalignment mark with a magnification which differs from the desiredmagnification and increases with an increasing wavelength difference.

To correct for said differences, the projection lens system PLincorporates an extra lens, a correction lens, 25. The correction lensis arranged at such a height in the projection lens that, on the onehand, in the plane of the correction lens the sub-beams of the differentdiffraction orders of the alignment beam, which sub-beams originatefrom, and are also generated by, a substrate alignment mark, aresufficiently separated to be able to influence these sub-beamsseparately and, on the other hand, this correction lens has a negligibleinfluence on the projection beam and the mask pattern image formed withit. The correction lens is preferably situated in the Fourier plane ofthe projection lens system. If, as is shown in FIG. 3, the correctionlens 25 is situated in a plane in which the chief rays of the alignmentbeams b and b′ intersect each other, then this lens can be used forcorrecting both alignment beams.

If desired, a wedge or a different deflection element such as adiffraction element may be arranged in the path of the alignment beam(s)proximate to an alignment mark. With such a deflection element, notshown in FIG. 3, alignment errors resulting from unintentional phasedifferences within the selected alignment beam portions received by thedetector 13 or 13′ may be prevented, which phase differences may occurif the axis of symmetry of the alignment beam portions coming from asubstrate alignment mark is not perpendicular to the mask plate, so thatfalse reflections may occur within this plate. An alignment unitprovided with such a deflection element is described in EP PatentApplication 0 467 445.

In addition to the global alignment marks P₁ and P₂, shown in FIG. 1,which are used for aligning the entire substrate with respect to themask, referred to as global alignment, the substrate may be providedwith further alignment marks per IC area so as to align each IC areaseparately with respect to the mask pattern. The mask may also comprisemore than two alignment marks in which the further alignment marks maybe used, for example to measure the rotation of the mask about the Zaxis so that this rotation can be corrected.

The projection apparatus further comprises a focus error detectionsystem for determining a deviation between the focal plane of theprojection lens system PL and the surface of the substrate W, so thatthis deviation can be corrected, for example by moving the projectionlens system along its axis, the Z axis. This system may be constitutedby the elements 40, 41, 42, 43, 44, 45 and 46 which are arranged in aholder (not shown) which is fixedly connected to the projection lenssystem. The reference numeral 40 denotes a radiation source, for examplea diode laser, which emits a focusing beam b₂. This beam is directed ata small angle onto the substrate by a reflecting prism 42. The beamreflected by the substrate is directed towards a retroreflector 44 bythe prism 43. The element 44 reflects the beam in itself so that thebeam (b₃′) once again traverses the same path via reflections on theprism 43, the substrate W and the prism 42.

The beam b₃′ reaches a radiation-sensitive detection system 46 via apartially reflecting element 41 and a reflecting element 45. Thisdetection system consists of, for example a position-dependent detector,or of two separate detectors. The position of the radiation spot formedby the beam b₃′ on this system is dependent on the extent to which thefocal plane of the projection lens system coincides with the plane ofthe substrate W. For an extensive description of the focus errordetection system, reference is made to U.S. Pat. No. 4,356,392.

For accurately determining the X and Y positions of the substrateholder, a stepping projection apparatus is provided with a multi-axisinterferometer system. U.S. Pat. No. 4,251,160 describes a system withtwo measuring axes and U.S. Pat. No. 4,737,283 describes a system withthree measuring axes. In FIG. 1, such an interferometer system isdiagrammatically shown by means of the elements 50, 51, 52 and 53, whileonly one measuring axis, the X axis, is shown. A beam b₄ emitted by aradiation source 50 in the form of a laser is split by a beam splitter51 into a measuring beam b_(4,m) and a reference beam b_(4,r). Themeasuring beam reaches a reflecting side face 54 of the substrate holderWH and the reflected measuring beam is combined by the beam splitter 51with the reference beam reflected by a stationary retroreflector, forexample a corner cube. The intensity of the combined beam is measuredwith a detector 53, and the displacement, in this case in the Xdirection, of the substrate holder WH can be derived from the outputsignal of this detector, and also an instantaneous position of thisholder can be determined.

As is diagrammatically shown in FIG. 1, the interferometer signals,represented for the sake of simplicity by one signal S₅₃, and thesignals S₁₃ and S′₁₃ of the double alignment unit are applied to asignal-processing unit SPU, for example a microcomputer, which processessaid signals to control signals S_(AC) for an actuator AC with which thesubstrate holder is moved in the XY plane via the substrate table WT.

With an interferometer system, which has not only the X measuring axisshown in FIG. 1 but also a Y measuring axis and possibly a thirdmeasuring axis, the positions of, and the mutual distances between, thealignment marks P₁, P₂ and M₁, M₂ can be fixed in a system ofcoordinates defined by the stationary interferometer system during theinitial, or global, alignment of the mask and the substrate with respectto each other. This interferometer system is also used for moving thesubstrate table very accurately, which is necessary for a steppingprojection apparatus so as to be able to step very accurately from afirst IC area to a second IC area.

If, as shown in FIG. 1, the projection apparatus is a step- and-scanapparatus, in which the mask and the substrate must be movedsynchronously during the projection of the mask pattern in an IC area,the mask must also be moved in one direction, the scanning direction.With the magnification M of the projection lens system being taken intoaccount, this movement must be synchronous with the correspondingmovement of the substrate. Then, the mask and the substrate must standstill with respect to each other during projection and both must bemoved with respect to the projection lens system and the projectionbeam. To measure the movement of the mask, the apparatus must beprovided with a second interferometer system. This interferometer systemcomprises the elements 60, 61, 62, 63 and 64 which have a similarfunction as the elements 50, 51, 52, 53 and 54. The signals from themask interferometer system, represented for the sake of simplicity by asignal S63 in FIG. 1, are applied to the signal-processing unit SPU inwhich these signals are compared with the corresponding signals from thesubstrate interferometer system. It can then be ascertained whether themask and the substrate mutually have the correct position and/or movesynchronously.

If the positions in the X and Y directions of the mask are representedby X_(r), Y_(r) and those of the substrate by X_(w), Y_(w) and therotation about the Z axis by φ_(r,r), and φ_(z,w), then the followingconditions are satisfied when the mask and the substrate are correctlypositioned with respect to each other:X _(x) −M.X _(r)=0  (1)Y _(w) −M.Y _(r)=0  (2)Φ_(z,w)−Φ_(z,r)=0  (3)

in which M is the magnification of the projection lens system. It hasbeen assumed that the mask and the substrate move in oppositedirections. If these elements move in the same direction, the minus signpreceding M in the above conditions should be replaced by a plus sign.

To ascertain whether these conditions have been met, it is sufficientthat both the interferometer system for the substrate and that for themask have three measuring axes.

However, the substrate interferometer system preferably has fivemeasuring axes. Then, not only X_(w), Y_(w) and φ_(z,w) but also φ_(x,w)and φ_(y,w), i.e. the tilts about the X axis and the Y axis can bemeasured.

To be able to measure such tilts of the mask, a five-axis maskinterferometer system may be used, or a combination of a three-axisinterferometer system for determining X_(r), Y_(r) and φ_(z,r) and othersensors such as capacitive sensors for the φ_(x,r) and φ_(y,r)measurements.

If X_(w), Y_(w), φ_(x,w), and φ_(y,w) and X_(r), Y_(r), φ_(z,r),φ_(x,r), φ_(y,r) and, with the aid of the focus error detection system,Z_(w) and Z_(r), i.e. the positions along the Z axis of the substrateand the mask can be measured, it can be ascertained whether not only theconditions (1), (2) and (3) are met, but also the conditions:M ² .Z _(w) −Z _(r)=0  (4)M.φ _(x,w)−φ_(x,r)=0  (5)M.φ _(y,w)−φ_(y,r)=0  (6)

The on-axis alignment unit, described with reference to FIG. 3, formutually aligning a mask alignment mark and a substrate alignment markwith respect to each other has been found to be eminently suitable forboth stepping and step-and-scan projection apparatuses with which imageshaving line widths up to a given minimal value are formed. However, itis expected that the use of novel technologies in the IC manufacture anddecreasing line widths in the images will lead to problems as far asaccuracy and reliability of the known alignment unit are concerned. Whenreducing the line width, the alignment accuracy must be enhanced. Whenusing said CMP process, asymmetries are introduced in the substrategrating mark so that the alignment procedure in which the first-ordersub-beams are used becomes unreliable. Moreover, when using an alignmentbeam having one wavelength, strict requirements must be imposed on thedepth of the grating grooves of the alignment mark, which requirementscan only be met with increasing difficulty.

All of these problems can be solved by making use of an off-axisalignment unit for aligning the substrate mark and by using higher-ordersub-beams, i.e. sub-beams having a diffraction order which is higherthan 1, in the alignment. Since the alignment of the substrate mark nolonger takes place through the projection lens system, there will begreater freedom to use more sub-beams, particularly higher-ordersub-beams. Since the resolving power of the alignment unit increaseswith an increasing order number of the sub-beams, the accuracy of thealignment can be enhanced considerably. Since notably the higher-ordersub-beams are determined by the edges of the substrate grating mark and,as compared with the center of the grating, these edges are lessinfluenced by said CMP process and other measures affecting the symmetryof the grating, the problem of asymmetry in the grating mark is largelyeliminated. Moreover, it is also possible to use alignment radiationwith more than one wavelength so that the requirements imposed on thedepth of the grating grooves can be alleviated considerably.

As will be elucidated hereinafter, the diffraction orders are separatedfrom each other by optical elements in the alignment unit according tothe invention, not by electronic means and/or associated software.Consequently, it is not necessary to measure signal amplitudes but thephase measurements which are more conventional in these kinds oftechniques can be used.

FIG. 4 is a schematic illustration of a lithographic system that has anoff-axis alignment system. The off-axis alignment system has two sourcesof radiation 70 for illuminating alignment marks at two differentwavelengths, for example a red laser and a green laser. Both lasersilluminate an alignment mark simultaneously and reflected light isdirected to separate detector channels (e.g., a red channel and a greenchannel). Signals in each of the two wavelength channels are thusobtained in parallel. In addition, several diffraction orders may beseparately detected for each of the two wavelengths, thus providing aplurality of color/order channels outputting signals in parallel. Aposition determining unit PDU is in communication with the plurality ofcolor/order channels of the detection system. The position determiningunit PDU may be a hard-wired special purpose device performingparticular functions or it may include a programmable computer that isprogrammed to perform the desired functions. In addition, it may be aseparate unit from the SPU illustrated in FIG. 1 or it may beimplemented through software in the SPU. The position determining unitPDU processes signals from at least two of the color/order channels todetermine the position of the alignment mark being detected.

FIG. 5 is a schematic illustration of an off-axis alignment unitaccording to an embodiment of this invention. Many structural featuresof alignment systems described here are similar or the same as thosedescribed in U.S. Pat. No. 6,297,876, the entire content of which isincorporated herein by reference. The substrate mark, in the form of agrating, is denoted by P₁. A parallel alignment beam b having awavelength; incident on this grating is split up into a number ofsub-beams extending at different angles α_(n) (not shown) to the normalon the grating, which angles are defined by the known grating formula:

$\begin{matrix}{{{Sin}\;\alpha_{n}} = \frac{N \cdot \lambda}{P}} & (7)\end{matrix}$

wherein N is the diffraction order number and P the grating period.

The path of the sub-beams reflected by the grating incorporates a lenssystem L₁ which converts the different directions of the sub-beams intodifferent positions u_(n) of these sub-beams in a plane 73:U _(n) =f ₁·α_(n)  (8)

In this plane, means are provided for further separating the differentsub-beams. To this end, a plate may be arranged in this plane, which isprovided with deflection elements in the form of, for example wedges. InFIG. 5, the wedge plate is denoted by WEP. The wedges are provided on,for example the rear side of the plate. A prism 72 can then be providedon the front side of the plate, with which an alignment beam coming fromthe radiation source 70, for example a He—Ne laser can be coupled intothe alignment unit. This prism can also prevent the 0-order sub-beamfrom reaching the detectors. The number of wedges corresponds to thenumber of sub-beams which is to be used. In the embodiment shown, thereare six wedges per dimension for the plus orders so that the sub-beamscan be used up to and including the 7-order for the alignment. Allwedges have a different wedge angle so that an optimal separation of thedifferent sub-beams is obtained.

A second lens system L₂ is arranged behind the wedge plate. This lenssystem images the mark P₁ in the plane of reference plate RGP. In theabsence of the wedge plate, all sub-beams would be superimposed in thereference plane. Since the different sub-beams through the wedge plateare deflected at different angles, the images formed by the sub-beamsreach different positions in the reference plane. These positions X_(n)are given by images formed by the sub-beams reach different positions inthe reference plane. These positions X_(n) are given byX _(n) =ƒ ₂·γ_(n)  (9)

in which γ is the angle at which a sub-beam is deflected by the wedgeplate.

At these positions, reference gratings G₉₀-G₉₆ can be provided, as isshown in FIG. 6. A separate detector 90-96 is arranged behind each ofthese reference gratings. The output signal of each detector isdependent on the extent to which the image of the substrate grating P₁coincides with the relevant reference grating. Hence, the extent ofalignment of the substrate grating, and thus of the substrate, can bemeasured with each detector 90-96. However, the accuracy with which themeasurement takes place is dependent on the order number of the sub-beamused; as this order number is larger, the accuracy is greater. In FIG.6, it has been assumed for the sake of simplicity that all referencegratings G₉₀-G₉₆ have the same grating period. Actually, however, thegrating period of each grating is adapted to the order number of theassociated sub-beam. As the order number is larger, the grating periodis smaller and a smaller alignment error can be detected.

Hitherto only one set of diffraction orders has been considered. As isknown, a diffraction grating forms, in addition to +1, +2, +3 etc ordersub-beams, also sub-beams of diffraction orders −1, −2, −3 etc. Both theplus orders and the minus orders sub-beams can be used to form thegrating image, i.e. a first image of the grating mark is formed by the+1 and −1 order sub-beams jointly, a second image is formed by the +2and −2 order sub-beams jointly, and so forth. For the +1 order and the−1 order sub-beams no wedges need to be used, but plane-parallel plateswhich compensate for path-length differences can be provided at thepositions of these sub-beams in the plane of the wedge plate. Thus sixwedges, both for the plus orders and for the minus orders, are requiredfor the orders 2-7.

FIG. 7 illustrates more clearly the functioning of the wedges of theembodiment of FIG. 5. In the more schematic FIG. 7, the first lenssystem L₁ and the second lens system L₂ are represented by wavy lines.For clearness sake only the sub-beams of the first orders b(+1) andb(−1), the sub-beams of the seventh order b(+7) and b(−7) and thesub-beams of another order b(+i) and b(−i), for example the fifth order,are shown. As FIG. 7 illustrates, the wedge angles, i.e. the angle whichthe inclined face of the wedge makes with the plane surface of the wedgeplate WEP, of the wedges 80 and 80′ are such that the sub-beams b(+7)and b(−7) are deflected in parallel directions and converged by thesecond lens system on one reference grating G₉₆. Also the sub-beamsb(+i) and b(−i) are deflected by the associated wedges 82 and 82′ inparallel directions and converged on one reference grating G₉₁. Thefirst order sub- beams are not deflected and are converged by the secondlens system on one reference grating G₉₃. By using both the plus orderand the minus order of each diffraction order a truthful image of thesubstrate grating mark P₁ is formed on the associated reference gratingand a maximum use is made of the available radiation.

FIG. 8 shows the preferred positions, with respect to the plane of themark P₁ and the reference grating plate RGP, of the lens systems L₁ andL₂ and the focal lengths of these lens systems. The lens system has afocal length f₁ and this system is arranged at a distance f₁ from theplane of the mark P. The lens system L₁ deflects the chief rays of thesub-beams in directions parallel to the optical axis OO′. The distancebetween the first lens system and the second lens system is equal tof₁+f₂ whereby f₂ is the focal length of the second lens system. Thereference grating plate is arranged at a distance f₂ from the secondlens system. As in the path between the two lens systems the chief raysof the sub-beams are parallel to the optical axis OO′, the position ofthe wedge plate is not critical.

In order that in the embodiment of FIG. 4 the plus- and minus ordersub-beams of the same diffraction order are deflected such that they canbe correctly superposed by the second lens system on the associatedreference grating, stringent requirements are to be set to the mutualquality of the two associated wedges. These quality requirements relateto the quality of the inclined faces of the wedges and to the wedgeangles.

To lessen said requirements and to release the tolerances of thealignment unit, preferably use is made of the structure of deflectionelements shown in FIG. 9. Instead of one discrete wedge for eachsub-beam a number of, for example three, wedge plates 190, 191, 192,which are common to all sub-beams, are used. FIG. 9 shows a perspectiveview and FIG. 10 a side view of the wedge plates. The wedge angle, i.e.the angle between the upper face and the lower face of a plate, forplate 192 the angle between the face 192 a and the face 192 b, aredifferent for the three plates. One of the plates, for example plate190, has a wedge angle which is opposite to those of the other plates.The plates are provided with a number of openings 200, only a few ofwhich are shown in FIG. 9. These openings are arranged at positionswhere sub-beams are incident on the relevant plate. However, not atevery such position an opening is present. If a sub-beam is incident onan opening in a plate it will not be deflected by this plate.

On its way through the plates a sub-beam will encounter, zero, one ortwo openings. Only the first order sub-beams encounters zero openingsand is not deflected by any of the plates. In FIG. 10 the path throughthe plates of one of the sub-beams is shown. This sub-beam is deflectedto the right by the first plate 190. Subsequently this sub-beam isdeflected over a smaller angle to the left. Finally this sub-beam passesthrough an opening 200 in the plate 192 so that no further deflectionoccurs. For each of the sub-beams the number of openings and the orderof the plate in which such opening is present is different from those ofthe other sub-beams, so that the sub-beams are all deflected indifferent directions. It will be clear that with a combination of threeplates 2³=8 different deflection directions can be realized. As a pairof sub-beams of the same diffraction order is deflected by the samewedge plates, the risk that these sub-beams are not deflected inparallel directions is minimized.

In the embodiment of FIGS. 5 and 6, sub-beams with an order number of 1to 7 are used so that seven reference gratings G₉₀-G₉₆ are necessary forthe alignment in the X direction. For the alignment in the Y direction,seven sub-beams may also be used together with seven further referencegratings G₉₃-G₁₀₄, as is shown in FIG. 6. A second series of twelvewedges is then arranged on the wedge plate in the Y direction in theembodiment of FIG. 5. In the embodiment of FIG. 9 a second series ofthree wedge plates is then arranged in the path of the sub-beams beforeor behind the first series of wedge plates, which second series ofplates deflect the sub-beams in Y-directions. The substrate mark may bethe mark shown in FIG. 2 or other types of marks, e.g., marks providedalong the scribe lines. For the first-order sub-beams, a similarreference grating may be used with four grating portions, two of whichhave a grating period of 16.0 μm, while the two other grating portionshave a period of 17.6 μm as is shown in FIG. 6. The other referencegratings have only one grating period which corresponds to the relevantdiffraction order of the grating portions with a period of 16 μm of thesubstrate grating P₁. The capture range of 44 μm associated with thegrating mark P₁ of FIG. 2 is then maintained.

In the embodiment of FIGS. 5 and 6, the sub-beams having the highestorder number are deflected by the deflection elements through thelargest angle. However, this is not necessary. Under circumstances, thisorder may be modified, for example for minimizing optical aberrations inthe grating images. That may also be the reason why the sub-beams withan ascending order number are deflected by the wedges alternately at apositive angle and a negative angle, as is shown in FIG. 6.

The minimum number of diffraction orders which has to be detected to beable to align in a sufficiently accurate way at a given asymmetry of thesubstrate mark P₁ can be determined by means of computer simulations.Such simulations have proved that, for example an alignment error of 150nm which remains when a first-order sub- beam is used can be reduced to20 nm when a fifth-order sub-beam is used.

In principle, the maximum number of orders which can be detected isdetermined by the minimum intensity which can still be detected and bythe numerical aperture of the lens system L₁, L₂. As is known, theintensity of the sub-beam formed by a diffraction grating quicklydecreases with an increase of the order number of this sub-beam; theintensity of a sub-beam is inversely proportional to the square of theorder number of this sub-beam. For a seventh-order sub-beam, theintensity is thus approximately 1/50 of that of a first-order sub-beam.The intensity loss due to reflections undergone by an alignment beamwhen traversing the off-axis alignment unit is, however, considerablysmaller than when it traverses an on-axis alignment unit. In thelast-mentioned unit, the alignment beam meets, for example approximatelyone hundred surfaces on which reflection losses may occur and in thefirst-mentioned unit it meets, for example only twenty of thesesurfaces. If the total reflection loss is a factor of four in theoff-axis alignment unit, the 7-order alignment sub-beam may have as muchintensity as a 1-order alignment beam in the on-axis alignment unit.

The numerical aperture NA_(n) which the optical system L₁, L₂ must haveto pass a sub-beam with a diffraction order of N is given by:

$\begin{matrix}{{NA}_{n} = {\sin\;{N \cdot \frac{ \lambda )}{P}}}} & (10)\end{matrix}$

For a 7-order sub-beam and a substrate grating mark with a gratingperiod p=16 μm and a wavelength λ=544 nm, the desired numerical apertureis approximately 0.24, which is a very acceptable number.

To guarantee a sufficiently stable system, the different referencegratings are provided on a single plate RGP which preferably consists ofquartz. The dimensions of this plate, hence the image field of thesecond lens system, are determined by the dimension d₁ of the referencegratings and their mutual distance d₂. This distance and dimension are,for example, both 0.2 mm so that the dimensions d_(x) and d_(y) in the Xand Y directions of the plate RGP are 2.8 mm and the desired fielddiameter is approximately 3 mm.

The discrete wedges of the embodiment of FIG. 5 may be made of glass orquartz and fixed to a quartz plate. This structure shows a high degreeof stability. The wedges may also be made of a transparent syntheticmaterial, for example an UV curable plastics. In that case it ispreferred to use a replication technique, known per se in optics, toprint the whole wedge structure by means of a mould in one run in a thinlayer of this material, which layer is applied to, for example a quartzsubstrate. As already remarked, instead of discrete wedges preferablywedge plates provided with openings are used. Instead of discrete wedgesor wedge plates other deflection elements may be alternatively used,such as diffraction gratings of which only one order is used.Furthermore it is possible to use deflection structures constituted bypatterns of refractive index variations in the material of a plate,which patterns are provided, for example by means of ion implantation.

In order that not too stringent requirements have to be imposed on thegroove depth of the substrate mark, alignment radiation having twowavelengths, for example 633 nm and 532 nm, have been found to besuitable. Use can be made of the fact that the angles at which thealignment grating deflects the sub-beams and the positions which thesebeams occupy in the rear focal plane of the lens system L₁ is dependenton the wavelength, as is apparent from the expressions (7) and (8). Inprinciple, the orders for the different wavelengths can be distinguishedfrom each other. Without further measures, however, a given order, forexample the second order of the first wavelength (633 nm) may comebetween, for example the second and third orders of the secondwavelength (532 nm). To separate the orders of the different wavelengthsbetter from each other, it can be ensured that the beams with thedifferent wavelengths are incident at different angles on the substrategrating P₁. For the case where seven diffraction orders are used, thesituation as shown in FIG. 11 is then created in the rear focal plane ofthe lens system L₁. Now, there is a first cross-shaped pattern ofpositions 110-137 for the different orders of the first wavelength and asecond cross-shaped pattern of positions 138-165 for the differentorders of the second wavelength. As is shown by means of the doublearrow in the center of FIG. 7, these patterns are offset with respect toeach other, which is due to the different angles of incidence of thealignment beams with the different wavelengths. These angles should bemaintained as minimal as possible so as to prevent alignment errorsoccurring due to defocusing effects. When using two wavelengths, theplate with deflection elements must of course be adapted to thesituation as is shown in FIG. 11, which means, inter alia, that insteadof 24 discrete wedges 48 wedges must be used, or that instead of 6wedge-shaped plates twelve of such plates must be used.

An alternative for the alignment with two wavelengths is illustrated inFIG. 12. In this figure, the reference numeral 160 denotes apolarization-sensitive beam splitter. This beam splitter receives afirst alignment beam b having a first wavelength λ₁, for example 633 nm,from a He—Ne laser, and having a first direction of polarization andpasses this beam to the substrate alignment mark P₁. Incident on thisbeam splitter is also a second alignment beam b₅ which has a secondwavelength λ₂, for example 532 nm and comes from a YAG laser preceding afrequency doubler. The beam b₅ has a direction of polarization which isperpendicular to that of the beam b so that the beam b₅ is reflected tothe substrate mark P₁. It has been ensured that the chief rays of thebeams b and b₅ are made to coincide by the beam splitter so that thesebeams will be passed as one beam to the mark P₁. After reflection by themark, the beams b and b₅ are split again by the beam splitter. Aseparate alignment unit 170, 180 is present for each of these beams.Each of these units emits an alignment beam and receives, via the beamsplitter, the sub-beams of the different diffraction orders coming fromthe substrate mark. In each of these units, images of the substrate markare formed on different reference gratings and with different sub-beams,as has been described with reference to FIG. 5. To this end, each unitis provided with a lens system L₁, L₂, (L₁′, L₂′), a wedge plate VVEP(WEP′) and FIG. 9 or a series of wedge-shaped plates, a plate withreference gratings RGP (RGP′), a number of detectors 90-96 (90′-96′) anda radiation source 70 (70′) whose beam is coupled into the system via acoupling prism 72 (72′).

FIG. 13 shows part of an embodiment of the alignment unit wherein aspecial kind of beamsplitter 160 is used. This beamsplitter comprises apolarization-sensitive beam splitting prism 210, a quarter-wave plate211 and a reflector 212. The beams b₁₀ and b₁₁ having differentwavelength and coming from sources, not shown, are indicated by thicklines and the beams reflected by the grating mark P₁ by thin lines. Thebeams b₁₀ and b₁₁ have the same polarization direction. The first beamb₁₀ is reflected by a reflector 215 towards the polarization-sensitivebeam-splitting layer 213 in the prism 210. This layer reflects the beamb₁₀ towards the grating mark P₁. The radiation reflected by the gratingmark and split up in sub-beams of different diffraction orders isrepresented by one single beam ray b₁₅. The beam b₁₅ is reflected by thelayer 213 towards the associated structure of deflection elements anddetectors which are not shown in FIG. 13.

The second beam b₁₁ is reflected by the reflector 216 towards thebeam-splitting layer 213 which reflects the beam towards thequarter-wave plate 212. After the beam b₁₁ has passed this plate it isreflected by the reflective layer 212 at the backside of this plate, sothat it passes for a second time through the plate 211. The beam b₁₂leaving the plate 211 has a polarization direction which is rotated over90° with respect to the polarization direction of the original beam b₁₁.The beam b₁₂ can pass the beam splitting layer 213 and reach the gratingmark P₁. The radiation reflected by this mark is also indicated by asingle beam ray b₁₆. This beam passes first the beam-splitting layer213, then traverses twice the quarter-wave plate 211 and finally isreflected by the layer 213 towards the associated structure of wedgesand detectors, not shown in FIG. 13. It is only for clearness sake thatin FIG. 13 the reflected beams b₁₆ and b₁₇ are represented as spatiallyseparated beams; in practice these beams coincide. The same holds forthe beams b₁₀ and b₁₁ at the position of the mark P₁.

In the embodiments of FIGS. 12 and 13 the first lens system L₁ ispreferably arranged between the beam splitter 216 and the grating markP₁, as shown in FIG. 13. This has the additional advantage that only onesuch lens system is needed for the two beams of different wavelengths.For the reflected beams separate second lens systems L₂, not shown inFIG. 13, remain necessary.

In the different embodiments described above the detectors are arrangeddirectly behind the reference gratings. In practice however, behind thereference gratings a bundle of imaging fibers may be arranged whichimage each of reference gratings and the superposed image of thesubstrate grating mark at a detector at a remote location, which is moreconvenient with respect to the design of the whole apparatus and withrespect to the performance of this apparatus. For example, cross-talkbetween the images formed by the sub-beams of the different diffractionorders can be decreased and heat generated by signal amplifiers andelectronic processors can be kept away from the alignment unit and theapparatus. Also the radiation sources may be arranged at positionsremote from the alignment unit and their radiation can also be guided tothe unit by an illumination bundle of fibers. In this way the heatgenerated by the radiation sources can be kept away from the alignmentunit and the projection apparatus.

Between the prism 216 and the second lens system L2 for one of the beamsb₁₅ and b₁₇ a partially transmitting reflector may be arranged to splitoff a portion of this beam towards a camera which, together with amonitor, provides a visual image of the substrate mark to an operator ofthe apparatus.

There are different possibilities of using the various detector signals.A start may be made with the alignment by means of the first-ordersub-beams by processing the signals of the detectors associated withthese sub-beams. Subsequently, the signals of the detectors associatedwith the second-order sub-beams may be used for finer alignment, thenthe signals of the detectors associated with the third-order sub-beamsmay be used for even finer alignment, and so forth. This may continue aslong as the sub-beams used still have sufficient intensity to bedetected in a reliable manner.

Another possibility is based on the recognition that the intensity ofcertain diffraction orders is increased at the expense of otherdiffraction orders when given process layers are provided on thesubstrate. In that case, a direct choice of the preferred orders may bemade for the alignment. Under circumstances, said possibilities may alsobe combined.

It is also possible to calibrate the alignment unit before a batch ofsubstrates is illuminated with a mask pattern or at the beginning of aproduction day. For a number of positions of the substrate mark thedetector signals for each of the diffraction orders are measured. Theresults of these measurements are stored in graphs or tables showing foreach position of the substrate mark the value of the detector signal foreach diffraction order. During illumination of the substrates, thealignment measurement can be performed by measuring only the relativelylarge detector signals of the lower diffraction orders, for example thefirst three orders. By interpolation the corresponding value for ahigher diffraction order, for example the seventh order, can bedetermined. In this way it is possible to determine alignment errorswith high resolution and large signal amplitude.

Hitherto, only the alignment of the substrate with respect to anapparatus reference in the form of reference gratings has beendescribed. With the same alignment unit also the position of thesubstrate holder or table can be determined. To that end this holder ortable is provided with an alignment mark similar to the substratealignment mark. (See, e.g., the fiducial mark illustrated schematicallyin FIG. 4.) The position of the substrate holder mark with respect tothe reference in the alignment unit is determined. The position of thesubstrate mark with respect to the substrate holder mark is then known.To be able to fix the mutual position of the mask pattern and thesubstrate, a further measurement is necessary, namely that of theposition of the mask pattern with respect to the substrate holder ortable. For this further measurement, the on-axis alignment unitdescribed with reference to FIGS. 1, 2 and 3 may be used, with whichmask marks are aligned with respect to marks of the substrate holder.Not only the double alignment unit as shown in FIG. 3 but also a singlealignment unit as described in U.S. Pat. No. 4,251,160 may be used.

Another possibility of aligning the mask pattern with respect to thesubstrate table is the use of the image sensor unit described in, forexample U.S. Pat. No. 4,540,277. In such a unit, a mask alignment markis imaged by means of projection radiation on a corresponding andtransmissive reference mark in the substrate table. In this table, adetector may be arranged behind the reference mark for converting theradiation passed by the reference mark into an electric signal. In thefirst instance, this image sensor unit is intended for, for examplecalibrating an on-axis alignment unit which operates with alignmentradiation having a wavelength which is considerably different from thatof the projection radiation, or for checking the image quality of theimage formed by the projection lens system and for measuring distortionsand aberrations which may occur, but it is also eminently suitable foraligning the mask pattern with respect to the substrate table. Insteadof the transmission image sensor unit described in U.S. Pat. No.4,540,277, an image sensor unit operating in reflection may bealternatively used for aligning a mask mark with respect to a substratetable mark. Such a unit, which is described in U.S. Pat. No. 5,144,363,operates with a reflective mark on the table and comprises a relativelylarge number of detectors which observe the mark at different angles andwhich, together with the associated optical systems, are provided in asensor plate which is arranged between the projection lens system andthe substrate table. The off-axis alignment unit according to theinvention must also be provided in this space. This unit must bearranged as close as possible to the center of the substrate table andrequires a building space which is conical with an aperture of, forexample 0.3. In practice, the length of the Y side of the substratetable approximately corresponds to the radius of the substrate for whichthe projection apparatus has been designed, for example 102 mm for an8-inch substrate, so that there is little room for building in thealignment unit in this direction. The X side of the substrate table is,however, for example 25 mm longer than the Y side, so that an alignmentunit which can handle an 8-inch substrate can be placed at a distance of25 mm from the optical axis of the projection lens system. This is shownvery diagrammatically in FIG. 14 which shows a part of the projectionlens system PL and its optical axis OO′. The portion between theprojection lens system and the substrate indicates the space which isoccupied by the projection beam, and the arrows marked b indicatesub-beams of the alignment radiation. The alignment beam is incident onthe substrate at a distance dx from the axis OO′ which distance is thus,for example 25 mm. The reference CS denotes the critical position forthe available building space. At this position, the diameter of the conewithin which the sub-beams with the different diffraction orders aresituated is equal to the distance to the substrate, multiplied by twicethe value of the numerical aperture. For a numerical aperture of 0.25and a value of 32 mm for said distance, said diameter, hence therequired vertical space at the location of CS, is 16 mm. This is areasonable requirement in practice. However, this vertical space may notbe completely available. In that case, two off-axis alignment units maybe used which are arranged diametrically with respect to each other andcan each cover a part of the substrate.

As hitherto described the off-axis alignment unit is arranged in theprojection column, comprising the mask holder, the projection system andthe substrate holder, of the lithographic projection apparatus. With theincreasing demand for larger IC's having smaller details, and thuscomprising more electronic components, the alignment procedure becomesmore and more time-consuming. The throughput of these apparatusestherefore tends to decrease without further measures taken. It hasalready been proposed to add to such an apparatus a separate measuringstation. In this station the position in, for example the X-, Y- andZ-direction, of a substrate is measured before this wafer is brought inthe projection column, or projection station. In the measuring stationsubstrate marks can be aligned with respect to alignment marks on thesubstrate holder or table. After the substrate, together with the holderhas been placed in the projection system only a mask alignment markneeds to be aligned with respect to the substrate holder mark, whichtakes only a short time. As in the apparatus, comprising a separatemeasuring station and projection station, during the illumination of afirst substrate in the projection station a second substrate is beingmeasured in the measurement station, the throughput of this apparatus isconsiderably larger than in an apparatus without a separate measurementstation. The alignment unit used in the measuring station for aligning asubstrate mark with respect to a substrate holder mark is preferably anoff-axis alignment system as described herein.

The off axis alignment systems described above are examples of alignmentsystems that have a plurality of sensors which produce a plurality ofsignals that can be combined to determine the position of an alignmentmark. In those examples, there are sensors that produce signals from thealignment mark for separate diffraction orders of light diffracted fromthe alignment mark. In the particular embodiments described, separateorders from the first order up to the seventh order can be detected.Furthermore, this is done for each of the X and Y directions. Inaddition, each of the seven orders in the X and Y directions can bedetected at two different wavelengths of light illuminating thealignment mark. Consequently, this provides a total of 28 channels thatprovide signals substantially simultaneously during an alignment scan.One could also receive signals that are switched in time, rather thanbeing simultaneous, without departing from the scope and spirit of thisinvention, especially if there is a rapid switching. According to theinstant invention, information from a plurality of channels of such amulti-sensor alignment system are combined to obtain improvedalignments. Improved alignment includes improved precision to align on asmaller scale and/or reduced errors due to systematic effects, such asprocessing effects, and/or improved reproducibility. It also includes“fallback procedures” in which alternative alignment steps or strategiesare available to replace a failed strategy. In an embodiment of thisinvention, signals from a plurality of different diffraction orders arecombined to determine a position of an alignment mark. In thisembodiment, detected positions of an alignment mark from a plurality ofdiffractive-order channels are fit to a curve which can be expressed asa continuous function and then this continuous function is used topredict a position that would be obtained at substantially no systematicerror.

An embodiment of such a predictive recipe has been found to be useful inthe case where marks are deformed, for example when a semiconductorwafer undergoes the tungsten chemical mechanical polishing process(W-CMP). (The aluminum physical vapor deposition (Al-PVD) process istypically performed in conjunction with W-CMP. Therefore, when we referto W-CMP one should understand that it can also include Al-PVD.) FIGS.15A-15D illustrate systematic effects that occur during the W-CMPprocessing which can lead to systematic errors in the detection of atarget on a wafer. FIG. 15A illustrates three series of grooves 310, 312and 314 etched into an oxide layer 316 which forms a multi-target mark,or a portion thereof. FIG. 15B shows a portion of the wafer after atungsten deposition step. A tungsten layer 318 is deposited over oxidelayer 316 as well as in grooves 310, 312 and 314. FIG. 15C shows thewafer after the W-CMP processing step. Finally, FIG. 15D illustrates aportion of the wafer after an aluminum layer 320 is deposited over thatportion of the wafer. As one can see in FIGS. 15C and 15D, there areasymmetric changes to the oxide layer, for example 322 and 324, that arethen also repeated in the aluminum layer, 326 and 328, due to the W-CMPprocessing. Such an asymmetric change to a diffraction grating that isused for an alignment mark leads to an apparent shift in the position ofthe alignment grating. This is a systematic effect introduced by theW-CMP processing step.

FIG. 16 is a plot of alignment errors introduced by the W-CMP processfor three different diffractive order channels. In this case, thedetected orders are the third, fifth and seventh diffractive-orderchannels. The data correspond to separate diffraction gratings for eachdetected order, as will be explained in more detail below. Such anembodiment has been found to provide good results, but the scope of thisinvention is not limited to only multi-grating marks. One can see thatthe error decreases with an increase in the detected diffraction order.The inventors found that a curve can be fit to such data and that thecurve can be extrapolated to predict reduced alignment error that onewould expect to see if the detection system were designed to detectcorresponding higher orders.

FIG. 17 illustrates the same data plotted a different way. The alignmenterrors for each of the three detected orders are now plotted as theinverse of the diffraction order multiplied by a constant factor (the“periodicity”). Since the plot is in terms of an inverseproportionality, as one goes to an infinite diffraction order, theposition in the graph would approach zero along the axis labeledperiodicity. Plotted in this way, one can see that the data can be fitwell by a straight line which approaches zero alignment error for thezero periodicity position. Therefore, by fitting a straight line to themeasured alignment errors for a plurality of diffractive order channels,one can extrapolate to predict the target position in the case of zeroalignment error which results from the W-CMP processing. The zeroperiodicity case corresponds to infinite diffractive order. Clearly, onecannot build a system that separately detects an infinite number ofdiffraction orders. However, the predictive method according to thisembodiment of the invention allows one to project to such a limit ofinfinite detected diffraction orders.

The inventors have found good results with reducing errors due to theW-CMP processing by combining signals from the third diffractive order,the fifth diffractive order and the seventh diffractive order in thefollowing linear combinationX _(pred)=−0.9399x ₃+0.6329x ₅+1.307x ₇,  (11)

which is based on a three point least squares fit to the followinggeneral equationx _(meas)(n)=C/n+x _(pred),  (12)

-   -   where C is a constant and n is the order number.

Although the inventors have found good results using only three of theseven diffractive orders, the broader concepts of the invention are notlimited to only the above-noted predictive recipe. Two or morediffractive-order channels may be used and measured values may be fit toother functions without departing from the general concept of thisinvention. Furthermore, this aspect of the invention is not limited toonly providing a predictive recipe to correct for effects introduced inthe W-CMP or similar processing.

In another example, the copper damascene process tends to introduceerrors which appear to be substantially random as observed in thedifferent wavelength and diffractive order channels. The inventors havefound that a predictive recipe in which multiple available signalchannels are averaged with equal weight tends to lead to good resultsfor the Cu-damascene process. General concepts of this invention includecombining information from a plurality of diffractive order channels toobtain an improved accuracy of detecting an alignment mark on asubstrate compared to using a single channel alone. Such predictiverecipes may take particular forms for particular processes performed onthe substrate as described above for the case of the W-CMP process andfor the Cu-damascene process. Furthermore, the concepts of thisinvention are not limited to only combining information from a case ofthe W-CMP process and for the Cu-damascene process. Furthermore, theconcepts of this invention are not limited to only combining informationfrom a plurality of wavelength channels and diffractive-order channelsto provide predictive recipes to account for processing effects on asubstrate. Information from a plurality of wavelength channels anddiffractive order channels may be combined to provide predictive recipesto account for other changes to a substrate that may lead to errors indetermining the position of an alignment mark.

The processing steps may introduce systematic effects that vary fromsubstrate to substrate. For example, an apparent expansion due to aW-CMP and Al-PVD processes is plotted in FIG. 18 as measured for thethird, fifth and seventh diffractive order channels for four differentsilicon wafers. The inventors found that such errors due to expansionand contraction vary with the detected diffraction order and also varyfrom substrate to substrate. As one can see in FIG. 18, the variationwith the periodicity, which is inversely proportional to the diffractiveorder, has a substantially linear variation for each wafer. However, thelines fit to the data for each wafer are different straight lines (e.g.,different slopes). Notice that the straight lines approach each otherfor a decrease in periodicity, corresponding to increasing diffractiveorder. Consequently, such a predictive recipe permits one to project toresults which are substantially invariant from wafer to wafer.Similarly, false rotations introduced by the W-CMP and AL-PVD processingcan be projected to decreased or substantially vanishing wafer to wafervariation. FIG. 19 provides data illustrating a predictive recipe forreducing wafer to wafer variations in this case.

The above examples of predictive recipes may be viewed as static recipesin the sense that the information from the plurality of channels arecombined with fixed coefficients. The term predictive recipe is intendedto include the general concept of obtaining a mathematicalrepresentation of multichannel information and then using themathematical representation to determine the position of the alignmentmark. The concepts of this invention also include dynamic recipes,within the general concept of predictive recipes, in which informationfrom the various channels are combined in a way that depends on measuredquantities. For example, information from a plurality of diffractionorder channels may be combined with coefficients which depend on themeasured signal strength. Other measured quantities may also be used indynamic recipes. For example, the output signal may be fit to anexpected functional form, such as a sinusoid. The correlationcoefficient in such a fit provides another measured quantity which canbe used in a dynamic recipe for combining signals from the plurality ofchannels. The use of other input parameters, such as “mccr”,“minirepro”, “signal to noise ratio” “signal shape”, “signal envelope”,“focus” “tilt” “order channels position offset” “wavelength channelsposition offset”, “shift between segments”; and/or “coarse-fine positiondeviation”, possibly in combination with user input parameters, canenhance the performance.

Many of these parameters are related to the accuracy of the alignedposition determination. The parameter “mcc” is the multiple correlationcoefficient indicating how well the measured signal resembles the signalexpected for a perfect alignment mark; the “minirepro” is the standarddeviation of the aligned position of different sections or portions ofan alignment measurement, indicating the accuracy of the alignedposition; the “signal to noise ratio” is the fitted signal divided bythe relative level of noise across the spectrum of the measured signal,while the “signal shape” is the relative level of a few discretefrequencies in this spectrum, generally at multiples of the basefrequency; the “signal envelope” is variance of the signal strengthduring the measurement; the “focus” is the offset in wafer height withrespect to the detector; the “tilt” is the angle between the wafer angleand the detector angle during the measurement; “order channels positionoffset” is the measured difference in aligned position of the variouschannels of one wavelength; the “wavelength channels position offset” isthe measured difference in aligned position of the various wavelengthchannels; the “shift between segments” is the measured difference inaligned position of the various segments of a multi segmented alignmentmark; and the “coarse-fine position deviation” is the difference betweenthe position of the alignment marks in the fine phase with respect totheir expected position based on alignment mark measurements in thecoarse phase.

The coefficients may also be determined by including the historical dataof the process. For example, one can compare the information obtainedfor the diffraction order channels with the information from previouswafers. If the information of a certain channel differs significantlyfrom the information for that channel on previous wafers, one could givethat channel a lower weighted coefficient than when the information fromthat channel closely resembles the information from previous wafers.Another way of dealing with the information from the plurality ofdiffraction order channels, is to model the individual channels in termsof wafer grid parameters (translation, rotation, wafer expansion,orthogonality, asymmetric scaling and higher order parameters) for eachof the channels. The residuals of the wafer model fit to the individualsignals—the so-called grid residuals—are qualifiers for the relativeimportance of a certain diffraction channel. For example, if a residualfor a channel on a certain position on the wafer closely resembles thehistorical residual distribution at that position, a larger weightfactor is assigned than when the residual is way off the averageresidual on previous wafers. (Also see FIG. 29 for an illustration of astatistical distribution of wafer residuals.) Historical data helps inthat way to minimize the spread of the information obtained with thealignment system. One can also measure the information from a pluralityof diffraction order channels and choose the channel(s) with the highestsignal strength(s) for alignment and reject the information ofdiffraction order channels with lower signal strengths. Other measuredquantities may also be used in such dynamic recipes.

Based on experimental data it is possible to determine the correlationbetween individual detectors. If the correlation is introduced by theprocess it can be utilized to generate weighting coefficients that arecapable of providing a more accurate measurement. One embodimentutilizes a set of data and determines a static predictive recipe basedon this information. A second embodiment verifies and adjusts apredictive recipe on the fly. Small adjustments are made to thepredictive recipe if the correlation between the detectors measured onthe current wafer are not identical to the predictive recipe.

In the examples of static recipes so far, the same information from theplurality of channels is used for all marks. Another type of staticrecipe can be distinguished that defines which information from theplurality of channels should be used for each target on the wafer. Sucha static recipe enables the alignment system to deal with processvariations across the surface. Instead of immediately selecting orweighting part of the information from the plurality of diffractionorder channels for each mark, it can also be useful to collect allinformation from all targets first.

Then the wafer grids can be determined for all of the individualchannels. Such a scenario offers much more flexibility. For instance, itis now possible to determine the wafer expansion on channels and/ormarks that are different from the channels/marks that are used todetermine the rotation.

The off-axial alignment system OAS may be used to detect various typesof alignment marks. FIG. 2 illustrates an alignment mark which is oftenused as a fiducial mark on the wafer assembly or along the periphery ofthe wafer as a global alignment mark. FIG. 20 illustrates a portion of asemiconductor wafer 400 with a plurality of regions such as 402 and 404where circuits will be, or are being, produced. Between the circuits arescribe lines, such as scribe lines 406 and 408. Alignment marks 410 arewritten in scribe line 408. Similarly, alignment marks 412 are writtenin scribe line 406. The alignment marks 410 and 412 are made to besufficiently narrow in order to maintain narrow scribe lines to avoidwaste of space on the wafer. The combination of alignment marks such as410 and 412 provide alignment information in orthogonal directions,which will be referred to as X and Y directions. The alignment mark 410is segmented such that it has a first target 414 and a second target416. Similarly, alignment mark 412 is segmented such that it has a firsttarget 418 and a second target 420. Each target 414, 416, 418, and 420is a diffraction grating in this embodiment. In general, the gratings414-420 can be either phase or amplitude gratings. For example, a phasegrating may be formed by etching grooves in the substrate or a layerthereon. In this embodiment, each of the diffraction grating targetswithin a respect mark has a different periodicity, or pitch. Undercurrent manufacturing processes and scales, it has been found to besuitable to use a pitch of 16.0 μm for one diffractive-grating target,such as target 416, and a 17.6 μm pitch for the seconddiffractive-grating target, such as target 414, and smaller. Such acombination is useful in the capture process that we will describe inmore detail below.

The capture process is a form of coarse alignment in which the positionof the alignment mark is established within a desired range. A finealignment is performed to determine a more precise position of thesubject alignment mark. During the alignment process, the wafer 400 willbe moved so that a desired alignment mark is scanned across thedetection field of view of the off-axial alignment system OAS in adirection substantially orthogonal to the grooves in the target gratingsof the alignment mark. Note that all signals for capture and finealignment can be obtained substantially in parallel. When the wafer 400is moved so that the target grating 416 of the alignment mark 410 movesacross the field of view of the alignment system OAS along the directionof the scribe line 408, an alignment beam having two wavelengthcomponents is reflected and diffracted from the target grating 416 (seealso FIG. 12). With the embodiment of the alignment system OAS describedabove, signals in seven diffraction order channels and two colorchannels are detected as the target grating 416 scans across the fieldof view of the alignment system OAS (see the reference grating plateillustrated in FIG. 6). There is also an additional channel in each ofthe X and Y directions and for each of the two colors for the captureprocess.

FIG. 21 illustrates an example of signals produced in the sevendiffraction-order channels 423A-423G for one of the two wavelengths as agrating, such as the target grating 416, is scanned at a constant rateunder the field of view of the off axial alignment system OAS. As theimage of the target grating 416 comes into alignment with the referencegrating for the respective diffractive order, there is a maximum in thesignal strength. Conversely, when the image of the target grating 416 onits respective reference grating is completely out of alignment, thereis a minimum in the detected signal strength. One can thus see that witha substantially constant scanning motion, the output signals aresubstantially sinusoidal. The signals for the higher order channels areat a higher frequency than the lower order channels. Signals areobtained in all seven of the diffractive order channels as a targetgrating 416 is scanned through the field of view of the off-axialalignment system OAS for each of the two colors of the alignment system.The alignment marks 410 and 412 provide one example of a multi-targetalignment mark. In this embodiment, the targets are diffractiongratings. The plurality of targets within the segmented alignment mark410 is used to determine a position of the alignment mark 410.Similarly, the targets within the segmented alignment mark 412 are usedto determine a position of the alignment mark 412. The concept of amulti-segmented alignment mark may be extended to include three, four ormore segments within an alignment mark. In addition to alignment markswith two segments, as illustrated in FIG. 20, the instant inventors havefound alignment marks that have four targets within the mark to becurrently useful. FIG. 22 illustrates an embodiment of such a fourtarget alignment mark 422 which has targets 424, 426, 428 and 430. Inthis embodiment, the targets 424, 426, 428 and 430 are each diffractiongratings and thus can also be referred to as target gratings. The targetgratings 424 , 426, 428 each have the same pitch, while the targetgrating 430 has a different pitch (not shown). Suitable pitches havebeen found to be 16.0 μm for the diffraction gratings 424, 426 and 428and 17.6 μm for the diffraction grating 430 for current feature scalesand some applications. In this embodiment, one may select each target424, 426 and 428 to have a different detection characteristic. Forexample, one may select the targets 424 and 426 and 428 to bediffraction order-enhancing gratings which enhance different diffractionorders. (In the foregoing, this means that it enhances the “signals” ofthat diffraction order compared to that of a uniform diffractiongrating.) For example, one may construct the alignment mark 422 suchthat the target 424 is a diffraction order-enhancing grating thatenhances the third diffraction order. The target 426 may be selected toenhance the fifth diffraction order and the target 428 may be selectedto enhance the seventh diffraction order.

FIGS. 23A-23D are schematic drawings of cross-sections of portions ofphase gratings. FIG. 23A corresponds to an ordinary diffraction gratingwith equally spaced grooves of equal widths. FIG. 23B is a schematicillustration of an order-enhancing grating for the third order in whichthe grooves between adjacent plateau regions in the ordinary grating ofFIG. 23A are now subdivided into two portions. The width of thissubstructure region is maintained substantially the same as the regions432, 434, and 436 illustrated in FIG. 23A. FIG. 23C illustrates anexample of an order-enhancing grating for the fifth order in which thesubstructure has three portions instead of the two portions of FIG. 23B.FIG. 23D illustrates an order-enhancing grating which enhances theseventh order. In the case of the 16.0 μm grating, each of thesub-segments 432, 434, 436 and the trough regions 438, 440, etc. are16.0 μm. The corresponding features in FIGS. 23B, 23C and 23D, are also16.0 μm, but some have substructures.

An order enhancing grating is defined as a grating with a signalstrength, in a diffraction order detection channel of the alignmentsensor, which is enhanced with respect to the signal strength detectedfor a base pitch grating. FIG. 23A is an example showing a base pitchgrating with a pitch of 16.0 μm. FIG. 23B, 23C and 23D are examples oforder enhancing gratings. A grating with a base pitch reduced by afactor N is another example of an order enhancing grating. The firstorder diffracted beam of such a grating will be detected by the N-thdiffraction order detection channel of the alignment sensor. Thisresults because a detector that we refer to as an nth-order detectordetects light coming from an angle that would coincide with the nthorder from a base pitch. Increasing or decreasing the pitch of thegrating changes the angles at which the light from the grating diverges,thereby potentially changing which order from the grating is actuallydetected by the nth order detector. The scope of this invention is notlimited to the above stated grating designs. All other gratings thatenhance a diffraction order are within the scope of this invention.

When the reference mark 422 is scanned across the field of view of theoff-axial alignment system OAS along its long dimension, the sensorproduces signals in seven diffractive-order channels for each of the twoillumination wavelengths. Consequently, a single scan across thealignment mark 422 can provide fourteen color/order signals for each ofthe targets 424, 426, 428 and 430. For an ordinary diffraction grating,the signal strength for the diffraction orders decreases with increasingdiffraction order. The order-enhancing gratings enhance a particulardiffraction order over what would normally be obtained with adiffraction grating with a purely constant pitch without anysubstructure. By selecting targets within the multi-target alignmentmark 422 to be particular order enhancing gratings, one may then chooseto utilize data of the enhanced order from the corresponding target. Forexample, when one selects target grating 424 to be a seventh orderenhancing grating, target 426 to be a fifth order enhancing grating, andtarget 428 to be a third order enhancing grating, the inventors haveobtained good results utilizing only the signals from the seventh orderchannels from the target 424, only the fifth order channels from thetarget 426, and only the third order channels from the target 428. Thedata illustrated in FIGS. 16-19 were obtained with such an alignmentmark and processed in that manner Although the data presented in FIGS.16-19 were obtained from signals in diffractive order channelscorresponding to an order enhancing target grating of the same order insuch a multi-target alignment mark, the general concepts described inreference to FIGS. 15-18 are not limited to only that case.

The alignment marks 410 and 412 in FIG. 20 provide an example of amulti-target alignment mark that has two targets that are diffractiongratings with different pitches. The four-target alignment mark 422 wasdescribed in terms of combining targets with both different pitches andtargets selected to enhance particular diffraction orders. One mayselect targets to achieve additional or different effects. For example,one may include a target among multi-target alignment marks that havestructural features that behave under processing in a predictable manneror in a manner correlated with changes in the devices beingmanufactured. For example, diffraction grating patterns may be providedwith a sub-pattern to provide it with desired characteristics undermanufacture. FIG. 24 illustrates an example in which a sub-pattern isadded to a portion of a diffraction grating. (See also FIG. 26C.) Forexample, the sub-structure 432 in FIG. 23A may be cut across to obtain astructure 442 illustrated schematically in FIG. 26C. One may choose thewidths of the structures in the component 442 so that they more closelyresemble the feature structures in a device that is being manufactured.The substructure illustrated in FIG. 24 is believed to be useful for theW-CMP process. As one can see in the right hand cross-sectional view ofFIG. 23, tungsten is used to fill the substructure grooves. One may alsointentionally create a sacrificial target among the plurality of targetswhich undergo significant changes and possibly are destroyed duringmanufacturing processes.

A goal of the sacrificial target(s) is to determine process dependenciesto improve the determination of the alignment mark position using thenon-sacrificial target(s). Therefore the sacrificial target(s) aresensitive to process influences. For example, a first target of amulti-target alignment mark is optimized for position stability, while asecond target of this multi target alignment mark is very sensitive to,for example, mark depth, line width or line angle. The sensitivity canbe used to correct the process influence on the first target, resultingin a more stable position. While the position of the first target isrelatively stable regardless of the process, the second target isoptimized to detect with a proper detector the process effects and withthis knowledge a better position of the alignment mark is calculatedbased on the combined information of both targets. An example of atarget useful for position stability is a target with a sectionimmediately above another section of the target. For example, thesections may be in different layers of a substrate. The differentsections of the target may be gratings with different pitches in oneexemplary embodiment. Since the target sections are at a nearlyidentical position relative to the plane of the substrate, signals fromboth target sections can be measured simultaneously. The signals fromthe different sections of the targets are thus not sensitive to errorsin the motion of the substrate or the substrate holder.

The goal of a sacrificial target in another embodiment is to prepare orfinish an alignment target positioned either above or below thesacrificial target. An example of a sacrificial target placed on top ofanother target is a structure which removes opaque material from thewafer surface. This enables light to penetrate the substrate and reachan underlying target, enabling alignment on a target which wouldotherwise not have been possible. An example of a sacrificial targetplaced beneath another target is an opaque structure. The effectivedepth of an alignment grating located in a transparent material can betuned by placing an opaque structure underneath the grating. The scopeof this embodiment is not limited to the examples provided. Otherstructures placed above or below an alignment target to improve thealignment performance are within the scope of this embodiment.

In another embodiment a first target closely resembles the productstructure and thus suffers substantially the same distortion and processeffects as the actual product. The target can be built up out ofsubstructures comprising features like contact holes or lines with aresolution more closely resembling the features in the product. Such atarget represents the position of the product better, but can also beaffected more by the process, making the target hard to detect. Forexample the CMP process is known to completely remove surface structuresat product resolution. A benefit of the multi target alignment mark isthat a robust second target is essentially always available for backuppurposes.

In another embodiment the overlay is determined using a multi targetalignment mark by printing target(s) or part of target(s) in differentlayers. An example is given using the Moire technique where in layer 1pattern a is printed and in layer 2 pattern b is printed overlappingpattern a, both forming a first target of a multi target alignment mark.Using a specific pattern a and pattern b, this generates a beat patternthat can be detected by the alignment sensor and the position shift ofthe first target with respect to a second target indicates the overlay.Benefits of the multi target mark in this matter are that the alignmenttargets are placed at substantially the same position on the wafer.Possible measurement errors due to process variations within a die willnot occur.

The sacrificial target can also be used for other purposes than positiondetermination, for example focus, energy, dose, line width, contact holewidth or critical dimension measurements and can be used insingle-target marks or multi-target marks. In this embodiment thesacrificial target is especially sensitive to these effects making themdetectable with an alignment or overlay sensor. Product line width,contact hole width or critical dimension generally can be measured usingthe relative signal strength of the alignment target when this targetconsists of groups of contact holes or lines with resolution and densitysimilar to the product. An example of an alignment target that issensitive to focus and dose is shown in WO 02/052350 A1, which is herebyincorporated by reference in its entirety. See also, EP-02253 1766, theentire content of which is hereby incorporated by reference. For thesemeasurements, some edge dies with no yield prospect are printed with afocus offset or dose offset and the focus or dose is detected with analignment or overlay sensor and adjusted in a feedback loop.

Using three detector channels, it becomes possible to do an onlinequalification and calibration of the measurement systems. For example,three detector channels measure the position of a multi target alignmentmark. With two detector channels agreeing on the measured position andthe third sensor measuring a different position this third sensor can bequalified as unreliable when measuring this alignment mark. In anotherexample two detector channels always measure a constant positiondifference in the aligned position, regardless of the process oralignment mark. This offset can be corrected by automatic calibration ormatching. In another example two detector channels have a specificoffset in position measurement for alignment marks on a particularprocess, this offset can be calibrated for said particular process. Thecalibrated offset can be used to correct the measured value whenswitching from one sensor to another or when both detector channels areused at the same time.

As noted above, the concept of multi-target alignment marks is notlimited to the use of only multi-grating alignment marks. The targetswithin the multi-target alignment mark may be targets of various types.It may include gratings as targets as well as other targets which aredetected by other processes, such as edge detection and/or imagerecognition techniques. In addition, some targets within a multi-targetalignment mark may be optimized for one particular type of sensor, suchas gratings are optimized for the off axial alignment system OASdescribed above, but other targets within the same mark may be suitablefor other alignment systems and/or other measurement systems. Forexample, a target within a multi-target alignment mark may be suitablefor detection by an online metrology tool that is used to assess theaccuracy of exposure of a photo-resist before further processing.

The capture process may also be improved by the use of new multi-targetalignment marks. Under the current capture process, the alignment sensoris scanned across a diffraction grating that has a first pitch. As notedabove, the scanning is typically accomplished by moving the substratethrough the field of view of the alignment sensor. This results in asinusoidal signal 500 of a first period, as is illustrated schematicallyin FIG. 25. The alignment sensor is then scanned across a diffractiongrating that has a second pitch that is greater than the first pitch, inthis example, in which the first and second gratings are part of thesame multi-target alignment mark, i.e., a single position will bedetermined for the alignment mark using the two targets in conjunction.A second sinusoidal signal 502 is generated from the second grating thathas a different period than that of the first signal. Consequently, apoint of coincidence of the maxima of the two signals 504 will come intocoincidence again after a desired number of cycles for appropriatelyselected pitches. For current applications, a first grating with a pitchof 16.0 μm and a second grating with a pitch of 17.6 μm have been foundto be useful. In this case, the signals come into coincidence againafter 11 cycles of the 16.0 μm grating and 10 cycles of the 17.6 μmgrating. This pattern repeats periodically, thus leading to ambiguity inthe correct alignment position (i.e., one may capture one of therepeating coinciding points. The capture range is ±44 μm in this case inwhich errors of multiples of 88 μm are introduced when the wrongambiguous ranges are chosen.

The capture process can also be performed using diffraction ordersub-beams detected with either of the axial or off-axial alignmentsystems described above. Furthermore, higher than first order sub-beamsmay be used for the capture process and/or diffraction order enhancinggratings may be used. In addition, a variety of types of targets may beused.

Other target types may also be included in a multi- target mark forperforming capture of the alignment mark. For example, one may includemultiple grating pairs in which each pair has a differentsub-segmentation, e.g., diffraction-order-enhanced gratings, ordifferent process segmentations, e.g., as discussed previously withreference to FIG. 24. FIGS. 26A and 26B illustrate two examples ofmultiple pairs of gratings. The alignment mark 600 has four targets 602,604, 606 and 608. The targets 602, 604 and 606 may be, for example,order enhancing gratings that enhance the third, fifth and seventhorders, respectively, and may have a pitch of 16.0 μm. (Note that thegratings that have a pitch of 16.0 μm are also referred to as 8.0 μmgratings because each line or groove, ignoring any substructure, isone-half the pitch.) The target 608 may be an order-enhancing gratingthat enhances the third order, for example, and has a pitch of 17.6 μm.Another example is the alignment mark 610 that has four order-enhancinggratings, all of which enhance the fifth order. The targets 612, 614 and616 each have a pitch of 16.0 μm, while the target 618 has a pitch of17.6 μm. In this example, the targets 614 and 616 may each have adifferent process segmentation. These are only a couple of specificexamples of the large number of possible variations. Which pair ofgratings is used for alignment can be pre-defined and/or can bedynamically determined based on the quality of the measured signals. Atypical qualifier for a pair of 16.0 and 17.6 μm signals is theremaining shift between two signal maxima of the closest 16.0 and 17.6μm maxima, zero remaining shift being the best. In addition,multi-target marks can be used in identifying particular detected marksthat are far from the mean value. If one uses three or more gratings,such as 16.0 μm gratings, or even arbitrary targets, one can compare thealigned positions of the gratings with each other and can detect if oneof the results is far from the other two or more. This deviatingalignment signal is called the “flyer” and will thus not be used fordetermining the position of the wafer. Flyers can be determined on amark-to-mark basis as a whole, or further specified with respect to aparticular detection channel. In addition, one can determine theposition on a target-to-target basis and targets as a whole areevaluated as to their position compared to the other targets of the markand then one can decide whether to keep the target for the alignedposition determination, or whether to reject the target. This mechanismhas been found to be especially useful during capture since theselection of the wrong maximum of a higher order (high frequency) afterthe capture process can be readily detected since the error will be atleast 8 μm (for 7th order) in a current example.

One of the multigrating embodiments is a capture multigrating mark. Thismark has two gratings optimized for capturing and two grating optimizedfor fine alignment. A major benefit of this embodiment is that acompromise is not required to enable both capturing and fine alignmentwith a single grating. Another embodiment is the process multigrating.(See, FIG. 26C.) This mark has one 17.6 μm pitch segment for capturingand three 16 μm pitch segments. The 16 μm pitch segments are equippedwith features enhancing the same diffraction order. The features of each16 μm pitch segment is optimized for a specific process window.Depending on the process characteristics one of the 16 micron pitchsegments will be used for alignment.

Another embodiment of this invention is a system for an improved overlaystrategy. The parameters for wafer alignment are important for anoverlay strategy in building up a micro-device. Some of the importantparameters are the number of alignment marks used, the alignment recipe,residual thresholds, and location of the marks on the wafer. FIG. 27illustrates a multi-target mark according to an embodiment of thisinvention that may be used with a system that has improved overlay. Thenumerical values of corresponding to the grating pitches (one-half thepitch values) are provided as an example of values that are currentlyfound to be useful and are not a limiting feature of the generalconcept. The alignment mark illustrated in FIG. 27 has three processtargets that provide robust alignment on such marks over most processescurrently used in a fabrication plant (“fab”) (i.e., CMP, PVD, STI, DT,Cu-Damascene, etc). Three process targets have been found to be useful,but one could use other numbers of process targets without departingfrom the scope of this invention. To enhance the robustness againstprocessing for a specific process, special process modules have beendesigned. Examples of process targets are described above with referenceto FIGS. 24 and 26C. For example, in cases where the W thickness variessignificantly during deposition, the process segmentations are chosen insuch a way that the full range of W-thickness variation is covered. Inthat way, optimal alignment performance after the W-CMP & AL-PVD processsteps is obtained.

One or more targets within a multi-target alignment mark are used foractive alignment while other targets are measured in parallel to provideadditional information such as the Signal Quality (MCC) which is acorrelation coefficient for the fit of the signal to an expectedfunctional form, signal strength (SS) and grid modeling parameters(Translation, Rotation, Magnification, etc). FIG. 28 is a schematicillustration the data flow in a fab-wide automatic process control (APC)system according to this invention. Since the APC system also receivesdata from off-line overlay metrology tools, the system can predictoverlays that could have been obtained with alternative strategies. TheAPC system can be configured according to this embodiment of theinvention to determine if and when a switch to a different strategy willbe made. Note that information can also be provided to a person to makea decision as to whether a change to a different strategy will be made.

The embodiment described immediately above is for a feedback loop in anexternal control system. This is useful for slowly varying parameterssuch as on a batch-to-batch basis. Another embodiment of this inventionprovides a “fallback” system and method, and dynamic global alignmentstrategies on wafer-to-wafer basis. This lithography-tool-based controlloop is an embodiment of Automated Equipment Control (AEC). In thisinvention, an automatic fallback is to one of the alternative strategiesin case the active alignment strategy fails. For instance, a predefinedfallback strategy or a strategy that performs second best can be usedfor fallback. In that way, the number of wafers that is rejected duringthe manufacturing process due to alignment errors can be minimized.

In case of alignment failure, it is clear that a switch to anotherstrategy should take place. However, in cases of dynamic globalalignment strategies within a batch, switching to an alternativestrategy is based on indirect indicators of overlay performance, sinceno external overlay data is available. Such indicators are for instancethe order-to-order stability, residual analysis or signal qualityanalysis (SS, MCC). See, “Extended Athena alignment performance andapplication for the 100 nm technology node” by Ramon Navarro, StefanKeij, Arie den Boef, Sicco Schets, Frank van Bilsen, Geert Simons, RonSchuurhuis, Jaap Burghoorn. 26th annual international symposium onmicrolithography, Feb. 25-Mar. 2, 2001 in Santa Clara, Calif., theentire content of which is hereby incorporated by reference.Order-to-order stability is a measure for the variation on the alignedposition as induced by processing only. Residual analysis characterizeshow well the modeled wafer grid fits into the measured positions. Thedecision to switch to an alternative strategy can be implemented withina batch. However, when switching to an alternative strategy, usually newprocess corrections are required. In both cases (fallback and dynamicglobal alignment strategies), the determination of the correct processcorrections is a problem. Since the process corrections are assumed tobe stable over batches, those can be derived in the slow feedback APCsystem. Thus, the APC system does not determine the parameters of thenew alignment strategy here, but determines the process corrections forall segments of the multi-target mark and sends those corrections to thelithography tool. With these data on the lithography tool, a feedbackloop can be implemented with switching of alignment strategy on a batch-to-batch basis.

In another embodiment of this invention, data collected from thealignment systems can be used to improve quality control duringmanufacture. Quality control is performed on an overlay metrology toolon a few wafers that are usually randomly picked from each batch.Therefore, it is very well possible that non-representative wafers arechosen from a batch. Since the process corrections for the next batchare based on these overlay metrology measurements, this may result inoverlay variations from batch-to-batch. In this embodiment, alignmentdata—that is available for every wafer—is used to identify the wafers tobe measured on the overlay metrology tool. To determine which wafers arerepresentative for a batch, one can for instance determine thewafer-to-wafer distribution of the wafer model parameters (translation,expansion, rotation) and identify the wafers that are closest to thebatch averaged wafer parameters. In particular, the wafer expansion andnon-orthogonality are useful for this purpose. Alternatively, one canlook at the grid residuals, i.e., the deviation of each measuredposition from a best grid fit to the measured positions. If thealignment marks have been exposed on a different machine, a systematicerror can occur with the grid residuals. The alignment system will thenmeasure each mark with a different offset. This offset gives a largecontribution to the residuals, thereby obscuring true processingeffects. By determining the residual distribution per mark location asshown in FIG. 27, the effect of the offset is excluded. Now it can bedetermined for all marks individually if they fit into the grid. Otherqualifiers that can be used are the SS and MCC distribution across allmarks and all wafers, or per mark across all wafers. Alternatively, onecould also indicate the worst wafer and suspect wafer (flyer). Forinstance, the worst SS, MCC, or residual may be used. If these worstwafers are within specification on the overlay metrology tool, theentire batch is also within specification.

In another embodiment of the invention, the alignment data is used todetermine the process corrections for the alternative strategies. Theprocess corrections are determined on the overlay metrology tool for theactive strategy only. The difference between the active and alternativestrategy is measured by the alignment system. The formula to calculateprocess corrections for a new strategy from the process corrections ofan old strategy and alignment data is as follows:PC ^(new) =<W ^(new) −W ^(old) >+PC ^(old).  (13)

This applies for each wafer model parameter, where PC is offline modeledwafer model parameters and W is the exposed wafer grid parameters. Theapplicable wafer parameters are: translation X,Y; wafer expansion X,Y;wafer rotation, non-orthogonality. Different scenario's are availablefor the averaging of alignment data, for example:

-   -   1. all wafers from the batch;    -   2. only the wafers used for offline overlay measurement;    -   3. previous batches (in case of rejects).

In another embodiment of this invention, instead of correcting themodeled overlay metrology data (PC), the raw overlay metrology data aregiven an offset according to the grid differenceW^(alternative)−W^(active) for each overlay target and each wafermeasured offline. Now the overlay performance and process correctionsfor that alternative strategy are calculated for the batch. The operatoror APC system can now monitor the trends in overlay (i.e., as a functionof time for several batches) so that the operator or APC systemdetermines whether to switch to one of the alternative strategies.

Since the numerous systems described above use coherent alignmentradiation sources, the phase modulation techniques described in U.S.Pat. No. 6,384,899 may also be used in combination with the systemsdescribed herein. The entire content of the U.S. Pat. No. 6,384,899 ishereby incorporated herein by reference in its entirety. The alignmentsystem according to the current invention may be implemented in avariety of alignment apparatuses. In a specific example, it may beimplemented in the alignment system illustrated in FIGS. 3, 5, 7, 12 and13. In this example, the alignment system has a position determiningunit. In general, the position determining unit may be either ahardwired special purpose component or may be a programmable component.In a programmable unit, the position determining unit comprises a CPU,memory and data storage area. In addition, the position determining unitwill have I/O ports for communicating with other equipment and/orinterfacing with users.

The invention is described with reference to exemplary embodiments. Itis not limited to only those embodiments and includes combinations andvariations of those embodiments within the scope of the invention asdefined by the appended claims. The invention is also described withreference to its use in apparatuses for step-and-scan imaging of a maskpattern on a substrate for manufacturing ICs, but this does not meanthat it is limited thereto. The invention may be alternatively used insuch an apparatus for manufacturing integrated, or plenary, opticalsystems, guidance and detection patterns for magnetic domain memories,or liquid crystalline display panels. The projection apparatus may notonly be an optical apparatus, in which the projection beam is a beam ofelectromagnetic radiation and the projection system is an opticalprojection lens system, but also an apparatus in which the projectionbeam is a charged-particle beam such as an electron beam, an ion beam oran X-ray beam, in which an associated projection system, for example anelectron lens system is used. Generally, the invention may be used inimaging systems with which images having very small details must beformed. Of course, various combinations of the above are also within thescope of this invention.

1. A method of automatic process control for the manufacture ofmicrodevices, comprising: receiving data from an alignment markdetection system having a plurality of detector channels; determining anupdated processing strategy based on said received data from saidalignment mark detection system; and altering a processing step based onsaid updated processing strategy, wherein said plurality of detectorchannels of said alignment mark detection system provide a correspondingplurality of signals substantially simultaneously during detection of analignment mark.
 2. A method of automatic process control according toclaim 1, wherein said data from said alignment mark detection system isobtained from a scan of a multitarget alignment mark.
 3. A method ofautomatic process control according to claim 2, wherein said multitargetalignment mark comprises at least two targets separately detectable bysaid alignment mark detection system to provide detection signals incorresponding at least two detector channels of said plurality ofdetector channels.
 4. A method of automatic process control according toclaim 3, wherein said at least two targets are eachdiffraction-order-enhancing gratings that enhance different diffractionorders, the corresponding at least two detector channels beingdiffraction-order channels.
 5. A method of automatic process controlaccording to claim 2, wherein at least one of said at least two targetsof said multitarget alignment mark is a process target that changes asignal characteristic in a substantially predictable manner during stepsof processing said microdevice.
 6. A method of automatic process controlaccording to claim 5, wherein said process target is a diffractiongrating having a substructure of cross trenches filled with tungsten. 7.A method of automatic process control according to claim 3, wherein atleast one of said at least two targets of said multitarget alignmentmark is a diffraction grating formed over a layer of material opaque toalignment radiation directed thereon, said layer of opaque materialacting to tune an effective depth of said diffraction grating.
 8. Amethod of automatic process control according to claim 2, wherein saidmultitarget alignment mark comprises at least four targets formed alonga scribe line of a semiconductor wafer.
 9. A method of automatic processcontrol according to claim 1, wherein said alignment mark detectionsystem is an off-axial alignment system of a lithographic exposureapparatus.
 10. A method of automatic process control according to claim9, further comprising: receiving data from an off-line metrology tool;and determining said updated processing strategy based on said receiveddata from said alignment mark detection system and said data from saidoff-line metrology tool.
 11. A method of automatic process controlaccording to claim 1, wherein at least two channels of said plurality ofdetector channels correspond to detection at different wavelengths. 12.An automatic process control system for the manufacture of microdevices,comprising: a data processing unity adapted to receive data from analignment mark detection system having a plurality of detector channels,wherein said data processing unit determines an updated processingstrategy based on data received from said alignment mark detectionsystem and outputs a signal to alter a processing step based on saidupdated processing strategy, and wherein said plurality of detectorchannels of said alignment mark detection system provide a correspondingplurality of signals substantially simultaneously during detection of analignment mark.
 13. An automatic process control system according toclaim 12, wherein said data from said alignment mark detection system isobtained during a scan of a multitarget alignment mark.
 14. An automaticprocess control system according to claim 13, wherein said multitargetalignment mark comprises at least two targets separately detectable bysaid alignment mark detection system to provide detection signals incorresponding at least two detector channels of said plurality ofdetector channels.
 15. An automatic process control system according toclaim 14, wherein said at least two targets are eachdiffraction-order-enhancing gratings that enhance different diffractionorders, the corresponding at least two detector channels beingdiffraction-order channels.
 16. An automatic process control systemaccording to claim 14, wherein at least one of said at least two targetsof said multitarget alignment mark is a process target that changes asignal characteristic in a substantially predictable manner during stepsof processing said microdevice.
 17. An automatic process control systemaccording to claim 16, wherein said process target is a diffractiongrating having a substructure of cross trenches filled with tungsten.18. An automatic process control system according to claim 14, whereinat least one of said at least two targets of said multitarget alignmentmark is a diffraction grating formed over a layer of material opaque toalignment radiation directed thereon, said layer of opaque materialacting to tune an effective depth of said diffraction grating.
 19. Anautomatic process control system according to claim 13, wherein saidmultitarget alignment mark comprises at least four targets formed alonga scribe line of a semiconductor wafer.
 20. An automatic process controlsystem according to claim 12, wherein said alignment mark detectionsystem is an off-axial alignment system of a lithographic exposureapparatus.
 21. An automatic process control system according to claim20, wherein said data processing unit is further adapted to receive datafrom an off-line metrology tool, and said data processing unitdetermines said updated processing strategy based on data received fromsaid alignment mark detection system and said off-line metrology tooland outputs said signal to alter said processing step based on saidupdated processing strategy.
 22. An automatic process control systemaccording to claim 12, wherein at least two channels of said pluralityof detector channels correspond to detection at different wavelengths.23. A method of automatic equipment control for the manufacture ofmicrodevices, comprising: receiving data from an alignment markdetection system having a plurality of detector channels; determining anupdated processing strategy based on said received data from saidalignment mark detection system; and altering a processing step based onsaid updated processing strategy, wherein said plurality of detectorchannels of said alignment mark detection system provide a correspondingplurality of signals substantially simultaneously during detection of analignment mark.
 24. A method of automatic equipment control according toclaim 23, wherein said data from said alignment mark detection system isobtained during a scan of a multitarget alignment mark.
 25. A method ofautomatic equipment control according to claim 24, wherein saidmultitarget alignment mark comprises at least two targets separatelydetectable by said alignment mark detection system to provide detectionsignals in corresponding at least two detector channels of saidplurality of detector channels.
 26. A method of automatic equipmentcontrol according to claim 25, wherein said at least two targets areeach diffraction-order-enhancing gratings that enhance differentdiffraction orders, the corresponding at least two detector channelsbeing diffraction-order channels.
 27. A method of automatic equipmentcontrol according to claim 26, wherein at least one of said at least twotargets of said multitarget alignment mark is a process target thatchanges a signal characteristic in a substantially predictable mannerduring steps of processing said microdevice.
 28. A method of automaticequipment control according to claim 27, wherein said process target isa diffraction grating having a substructure of cross trenches filledwith tungsten.
 29. A method of automatic equipment control according toclaim 25, wherein at least one of said at least two targets of saidmultitarget alignment mark is a diffraction grating formed over a layerof material opaque to alignment radiation directed thereon, said layerof opaque material acting to tune an effective depth of said diffractiongrating.
 30. A method of automatic equipment control according to claim24, wherein said multitarget alignment mark comprises at least fourtargets formed along a scribe line of a semiconductor wafer.
 31. Amethod of automatic equipment control according to claim 23, whereinsaid alignment mark detection system is an off-axial alignment system ofa lithographic exposure apparatus.
 32. A method of automatic equipmentcontrol according to claim 23, wherein at least two channels of saidplurality of detector channels correspond to detection at differentwavelengths.